it's too late for you when you feel thirsty. You need to drink exactly 8 glasses of water per day. Sounds familiar?
These are the usual advices when it comes to hydration, either for the general population or the athletes. In the following article I'll write about hydration for athletes. Just like when we talk about healthy diet, the differences between athletes and the general population do exist. But, they are not that big and they are mostly about timing and supplementation.
This article will be somewhat longer than the others. I'd like to point out it does not mean that hydration is more important than other diet components. It is a topic I earned my master degree on, researched it in huge detail, and it would be a shame not to share it with you. If it's too long for you, you can find the summary at the end of the article as always.
Metabolism, nutrients transport, circulation, temperature regulation, muscle contraction, nerve impulse transport, maintaining the electrolytes balance, and getting rid of waste wouldn't be possible without water because it is a medium for all cellular and systemic processes in the body. 
Water balance is achieved when the water intake from food and fluids is equal to its losses which occur mostly through sweating (500 mL), urinating (1400 mL), feces (200 mL) and breathing (400 mL).  The balance is maintained through the feeling of thirst, which is regulated via central and peripheral mechanisms. Most healthy people are able to ensure they're hydrated just by relying on feeling of thirst, which does not apply to sick people, elderly, newborns, soldiers and athletes. For these groups, fluid intake generally needs to be planned, and cannot rely solely on feeling of thirst.
The level of hydration is not simple to measure, and there is no agreement which way is the best way, especially outside of the laboratory.  Tracking the acute changes in body weight, as well as colour of morning urine is an adequate and practical way to self-estimate the level of hydration. Morning body weight 1% lower than usual, combined with dark color of urine and feeling of thirst, suggests dehydration. 
Loss of bodily fluids, if serious enough, can compromise many physiological functions, like cardiovascular and kidney functions, and can also cause weakness, headache, nausea, and all-around fatigue. But we must keep in mind that the water content fluctuates on a daily basis with no consequences on health and behavior. The point in which dehydration signs become obvious is debatable, due to many different methodologies used for inducing dehydration in research subject and their different characteristics.  Low water intake is related to the occurrence of coronary heart disease, bladder carcinoma, and frequent urinary tract infections. [5,6,7] Even though kidneys have the ability to concentrate and dilute urine, depending on water intake, they require more energy when the inatke is insufficient, and kidney tissue wears down. 
Other than physiological functions, dehydration has a negative effect on cognitive functions as well. We don't know which functions exactly are sensitive to dehydration.  Some authors set the line of negative effect at long term lack of fluids of around 1,5% of body weight. 
Hydration for athletes
As I already mentioned, the level of hydration affects the majority of physiological and metabolic functions. This is primarily the case with cardiovascular and thermoregulatory functions which are closely related to sports performance.  People engage in sports activities during all kinds of weather conditions (temperature, humidity, exposure to sun and wind etc.). Depending on these, intensity and duration of activity, the rise in body temperature can be significant. Research suggests that fatigue occurs when body temperature reaches 40°C.  The body is trying to get rid of excess heat by increased flow of blood through the skin (vasodilatation) and sweating.  Sweating can reduce body temperature and prolong the maximal duration of activity, but the consequences are loss of bodily fluids and electrolytes, mostly sodium. Loss of water results in reduced blood volume that body adjusts to by cardiovascular changes such as increased heart rate.  These adjustments cannot prevent performance loss long-term.
If they're not properly restored, fluids and electrolytes losses can compromise the performance in activities of longer duration, and possibly have negative impact on health as well. The level of sweating depends on the intensity of activity, its duration, athlete's equipment, level of hydration, weather conditions, and acclimatization to them.  During the high intensity activities, high temperatures, and high humidity, sweat levels have been recorded to be 3 L/h. The highest recorded level was 3,71 L/h. 
Even though athletes diet needs to be different for recreational and competitive athletes, proper hydration remains an important factor. It does not affect only performance but health as well. In recreational sports, due to moderate duration and intensity, losses rarely surpass 2% of body weight so rehydration during the activity is not crucial. Even though many believe the opposite, recreational athletes don't need to spend money on sports beverages containing sodium because their trainings aren't that intense or long for significant losses to occur.
Effect of dehydration on sports performance
Most authors agree that in warm conditions (> 30°C) aerobic sports performance becomes compromised at dehydration of 2-3% of body weight. The level up to which it is compromised is varies from 7 to 60%.  While the negative influence of dehydration to aerobic performance is clearly visible in warm conditions, it is not so noticeable in moderate and cold conditions, and neither is it clear when it comes to anaerobic performance. No negative influence has been noticed in cold conditions for performance shorter than 90 min in dehydration levels of 3%, , and it seems that dehydration up to 7% has no influence on anaerobic performance. 
Dehydration, other than compromising aerobic performance, increases the risk of heat exhaustion, , and it poses a risk for heat stroke.  Physiological factors that induce drop in performance caused by dehydration are increased thermal stress, increased cardiovascular effort, increased glycogen consumption, and possibly moderated function of the central nervous system which manifests in enhanced sense of fatigue, and drop in motivation.  It is believed that muscle cramps are connected with dehydration and electrolyte deficiency, even though this connection is more anecdotal than founded in solid proof.  Each factor is specific and independent, but it seems that the negative performance effect is the consequence of their interaction.  Dehydration has a negative effect on cognitive performance such as concentration, caution and short-term memory, necessary in sports requiring a certain level of concentration. 
There is a significant difference in dehydration consequences when it comes to duration of the activity. The effect on longer activities are relatively well documented, but the effect of dehydration on shorter activities is not clear. Some authors claim that a certain level of dehydration could even be desirable in certain situations like high jump where a lower body weight is desirable (with retaining strength and explosiveness, which dehydration does not affect). 
Lately the line over which we can see negative effects on aerobic performance has been researched because older recommendations were mostly based on laboratory measurements, and not real training and competition conditions. Newer analyses demonstrate that athletes who observe the feeling of thirst have the best results.  It's very interesting to find out that marathon time is in inverse correlation with body weight loss during the race.  Same was determined for Ironman triathlon  and 24-hour ultramarathon.  The winner of Dubai marathon in 2009, Haile Gebrselassie, finished the race with body weight loss of 9,8%.  Results of meta-analyses on this topic determine that body weight loss due to dehydration has a beneficial effect on performance – however, not a statistically significant one. 
Recommendations for hydration before the activity
Every athlete, recreational or professional, should strive to begin the activity adequately hydrated with normal levels of electrolytes in blood plasma. This way he or she reduces the risk of dehydration during activity. To ensure that, the focus should be put on feeling of thirst and urine color during the day, and react on time by sufficient fluid intake. On time means up to 2 hours before the activity, in order to avoid stopping the activity because of the need for urination. Hydration before activity overlaps with rehydration.
Recommendations for hydration during the activity
The goal during the activity is to prevent the dehydration that would compromise the performance. People usually don't ingest enough fluids during the activity to replenish the fluids lost through sweat. This is known as voluntary dehydration and its influence on performance, as I mentioned before, is not as grand as was thought. The explanation is two-sided: either the dehydration in race conditions doesn't compromise the performance as was previously considered, or the lower energy consumption due to sweat-induced loss of body weight that the athlete needs to move compensates for the negative effects of dehydration.
Due to significant interindividual differences in the quantity of sweat and electrolyte concentration in it, which also depend on weather conditions, it's recommended to make an individual plan of hydration. The basics for this are weighing yourself before and after the activity in different weather conditions (temperature, humidity) and exercise conditions (intensity). The plan includes the quantity and timing fluid intake, and if carbohydrates are put into the equation, also their quantity and concentration which should be in line with recommendations.
Fluid intake during the activity is a chance, for athletes engaging in activities longer than 1 hour, to ingest carbohydrates as well. In activities shorter than 1 hour, fluid or carb intake is not necessary, but athletes might gain benefits from drinking a small portion of fluids with carbs or just washing your mouth with it, which has been proven to have a positive effect on performance. Probably because of central nervous system stimulation and motivation.
In short, fluid intake during the activity should satisfy these goals:
retain the loss of body weight in 2-4% range
not cause the need for urination
in activities longer than 1 hour, become the medium for carb intake
In order to achieve all of this, different ways of intake should be tried out during training, and NOT during competition.
Electrolyte intake during the activity, mostly sodium, makes sense only in activities longer than 3-4 hours, when we sweat profusely.
Recommendations for hydration after the activity
The goal of rehydration is primarily to make up for fluid loss, and secondarily for electrolyte and carbohydrate loss, that occurred during the activity. Rehydration after the activity is at the same time hydration before the next activity. If there's enough time, thirst is a good enough incentive to consume enough water in order to replenish the loss. Athletes involved in more than one training per day, where the distance between the two isn't long enough to replenish the loss with the usual diet, should do this more aggressively.  It's recommended to take in the amount of fluids equal to 150% of lost body weight during the activity – several hours after the activity ended. 
Due to strong diuretic effect, strong alcoholic drinks should be avoided in the period around the training or race, but there's no evidence that a moderate intake during the day, if a person drinks enough fluids, would harm the level of hydration.  Talking about diuretics, I'd mention caffeine that many falsely consider a strong diuretic - research does not support that. A short term effect on urine production is present, but reduced during the day, so the difference between caffeine consumers and non-consumers disappears. 
Loss of electrolytes
The content of human sweat differs among individuals. That's why the needs for electrolyte replenishment are different, and sodium takes a special place. People that are especially at risk are those who sweat profusely, at the same time the electrolyte contents in their sweat are high, and they consciously restrict dietary intake of salt because they believe it has a negative effect on cardiovascular health.
Regular diet without the significant salt restriction (sodium chloride) will be enough to compensate for the loss. Also, consuming beverages that contain sodium and eating meals with salt will stimulate thirst and help retain the fluids consumed.  I'm not saying you should pour salt on every bite you take, but don't refrain from it in your daily diet.
Significant dehydration can cause health issues. It is less known that excess hydration can do the same. Even though dehydration is more common, hyperhydration leading to hyponatremia is even more dangerous, because it can lead to coma and death faster than dehydration. Drop in sodium concentration below 120 mmol/L increases the risk of serious cerebral edema with the onset of coma, breathing failure, and death.  The most important factor in hyponatremia occurrence is ingesting too much hypotonic fluids (without electrolytes), but it can also occur due to high unreplenished sodium loss through sweating during ultra endurance sports.  Since hyponatremia occurs mostly in activities longer than 4 hours, it probably won't be widespread between the general population of recreational athletes who are active for 2 or less hours per day.  The simplest recommendation to avoid hyponatremia is to not force yourself to drink. But if you do, drink beverages with electrolytes.
When trying to optimize the level of hydration, athletes might experiment with alternative methods. One of them is hyperhydration, a short-term (during the competition) increase in bodily water storage to make it higher than usual. Most commonly used way to achieve hyperhydration is glycerol consumption. However, hyperhydration using glycerol significantly increases the need of urination during the activity, , and even though the research isn't unanimous, it seems to not have a physiological advantage over the usual hydration.  Also, hyper hydration can lower the sodium concentration in blood plasma even before the activity begins,  which increases the risk of hyponatremia if the levels of fluid intake with no sodium are high during the activity.  It's useful to mention that glycerol is on the doping list of WADA (World Anti-Doping Agency) since 2010.
Hyperhydration can be achieved through increased intake of sodium – salt – in a short period before the competition, but in this case no increase in sports performance was documented. 
DURING THE DAY AND BEFORE THE ACTIVITY
The quantity of fluid that will avoid feeling of thirst and produce a light urine color; replenishing 500 mL of fluids between 2 and 3 hours before the activity
DURING THE ACTIVITY
Individual; limiting the loss of body weight to 2-5%, regular periods of intake
 Armstrong, L. E. (2010) Rationale for Renewed Emphasis on Dietary Water Intake. Nutr. Today. 45, S4–S6.
 Malisova, O., Bountziouka, V., Panagiotakos, D. Β. (2013) Evaluation of seasonality on total water intake, water loss and water balance in the general population in Greece. J. Hum. Nutr. Diet. 26, S1, 90-96.
 Kenefick, R. W., Cheuvront, S. N. (2012) Hydration for recreational sport and physical activity. Nutr. Rev. 70, S2, S137–142.
 Maughan, R. J. (2012) Investigating the associations between hydration and exercise performance: methodology and limitations. Nutr. Rev, 70, S2, S128–S131.
 Chan, J., Knutsen, S. F., Blix, G. G. (2002) Water, other fluids, and fatal coronary heart disease: the Adventist health study. Am. J. Epidemiol. 155, 827-833.
 Altieri, A., La Vecchia, C., Negri, E. (2003) Fluid intake and risk of bladder and other cancers. Eur. J. Clin. Nutr. 57 S2, S59-S68.
 Tack, I. (2010) Effects of Water Consumption on Kidney Function and Excretion. Nutr. Today 45, S37-S40.
 Popkin, B. M., D’Anci, K. E., Rosenberg, I. H. (2010) Water, hydration, and health. Nutr. Rev. 68, 439–458.
 Lieberman, H. R. (2007) Hydration and cognition: a critical review and recommendation for future research. J. Am. Coll. Nutr. 26, S5, 555-561.
 Szinnai, G., Schachinger, H., Arnaud, M. J. (2005) Effect of water deprivation on cognitive-motor performance in healthy men and women. Am. J. Physiol. Regul. Integr. Comp. Physiol. 289, 275–280.
 Fortney, S. M., Vroman, N. B., Beckett, W. S. (1988) Effect of exercise hemoconcentration and hyperosmolality on exercise responses. J. Appl. Physiol. 65, 519-524.
 Nielsen, B., Strange, S., Christensen, N. J. (1997) Acute and adaptive responses in humans to exercise in a warm, humid environment. Pflugers Arch. 434, 49–56.
 Sawka, M. N., Wenger, C. B., Pandolf, K. B. (1996) Thermoregulatory Responses to Acute Exercise-Heat Stress and Heat Acclimation. Compr. Physiol. S14: Handbook of Physiology, Environmental Physiology: 157-185.
 Maughan, R., Shirreffs, S. (2004) Exercise in the heat: challenges and opportunities. J. Sports Sci. 22, 917–927.
 Shapiro, Y., Pandolf, K. B., Goldman, R.F. (1982) Predicting sweat loss response to exercise, environment and clothing. Eur. J. Appl. Physiol. 48, 83–96.
 Rehrer, N. J. (2001) Fluid and electrolyte balance in ultra-endurance sport. Sports Med. 31, 701–715.
 Armstrong, L. E., Hubbard, R. W., Jones, B. H. (1986). Preparing Alberto Salazar for the heat of the 1984 Olympic marathon. Phys. Sportsmed. 14, 73-81.
 Cheuvront, S. N., Carter. R. 3rd, Castellani, J. W. (2005) Hypohydration impairs endurance exercise performance in temperate but not cold air. J. Appl. Physiol. 99, 1972–1976.
 Cheuvront, S. N., Carter, R. 3rd, Haymes, E. M. (2006) No effect of moderate hypohydration or hyperthermia on anaerobic exercise performance. Med. Sci. Sports Exerc. 38:1093–1097.
 McLellan, T. M., Cheung, S. S., Latzka, W. A. (1999) Effects of dehydration, hypohydration, and hyperhydration on tolerance during uncompensable heat stress. Can. J. Appl. Physiol. 24, 349–361.
 Carter, R. I., Cheuvront, S. N., Williams, J. O. (2005) Hospitalizations and death from heat illness in US Army soldiers, 1980–2002. Med. Sci. Sports Exerc. 37, 1338–1344.
 Nybo, L., Nielsen, B. (2001) Hyperthermia and central fatigue during prolonged exercise in humans. J. Appl. Physiol. 91, 1055–1060.
 Braulick, K. W., Miller, K. C., Albrecht, J. M. (2013) Significant and serious dehydration does not affect skeletal muscle cramp threshold frequency. Br. J. Sports Med. 47, 710-714.
 Cheuvront, S. N., Carter, R. 3rd, Montain, S. J. (2004) Influence of hydration and air flow on thermoregulatory control in the heat. J. Therm. Biol. 29, 532–540.
 Hancock, P. A., Vasmatzidis, I. (2003) Effects of heat stress on cognitive performance: the current state of knowledge. Int. J. Hyperthermia. 19, 355-372.
 Goulet, E. D. (2012) Dehydration and endurance performance in competitive athletes. Nutr. Rev. 70, S2, S132–136.
 Zouhal, H., Groussard, C., Minter, G. (2011) Inverse relationship between percentage body weight change and finishing time in 643 forty-two-kilometre marathon runners. Br. J. Sports Med. 45, 1101–1105.
 Sharwood, K. A., Collins, M., Goedecke, J.H. (2004) Weight changes, medical complications, and performance during an Ironman triathlon. Br. J. Sports Med. 38, 718–724.
 Kao, W. F., Shyu, C. L., Yang, X. W. (2008) Athletic performance and serial weight changes during 12- and 24-hour ultra-marathons. Clin. J. Sport Med. 18, 155–158.
 Beis, L.Y., Wright-Whyte, M., Fudge, B. (2012) Drinking behaviors of elite male runners during marathon competition. Clin. J. Sport Med. 22, 254–261.
 Goulet, E. D. (2011) Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis. Br. J. Sports Med. 45, 1149–1156.
 Shirreffs, S. M., Maughan, R. J. (1998) Volume repletion after exercise-induced volume depletion in humans: replacement of water and sodium losses. Am. J. Physiol. 274, 868–875.
 Godek, S. F., Bartolozzi, A. R., Godek, J. J. (2005) Sweat rate and fluid turnover in American football players compared with runners in a hot and humid environment. Br. J. Sports Med. 39, 205–211.
 Costill, D. L., Sparks, K. E. (1973) Rapid fluid replacement following thermal dehydration. J. Appl. Physiol. 34, 299–303.
 Shirreffs, S. M. (2001) Post-exercise rehydration and recovery. U: Sports Drinks, (Maughan, R. J., Murray, R., ured) Boca Raton, FL: CRC Press, str. 183-196.
 Maughan, R. J., Griffin, J. (2003) Caffeine ingestion and fluid balance: a review. J. Hum. Nutr. Diet. 16, 411–420.
 Shirreffs, S. M., Taylor, A. J., Leiper, J. B. (1996) Post-exercise rehydration in man: effects of volume consumed and sodium content of ingested fluids. Med. Sci. Sports Exerc. 28, 1260–1271.
 Murray, B., Eichner, E. R.. Hyponatremia of exercise. Curr. Sports Med. Rep. 3, 117–118.
 Montain, S. J., Cheuvront S. N., Sawka, M. N. (2006) Exerciseassociated hyponatremia: quantitative analysis for understand the aetiology. Br. J. Sports Med. 40, 98–106.
 Sharp, R. L. (2006) Role of sodium in fluid homeostasis with exercise. J. Am. Coll. Nutr. 25, S3, 231S–239S.
 Freund, B. J., Montain, S. J., Young, A. J. (1995) Glycerol hyperhydration: hormonal, renal, and vascular fluid responses. J. Appl. Physiol. 79, 2069–2077.
 Kavouras, S. A., Armstrong, L. E., Maresh, C. M. (2005) Rehydration with glycerol: endocrine, cardiovascular and thermoregulatory responses during exercise in heat. J. Appl. Physiol. 100, 442-450.
 O'Brien, C., Freund, B. J., Young, A. J. (2005) Glycerol hyperhydration: physiological responses during coldair exposure. J. Appl. Physiol. 99, 515–521.
 Montain, S. J., Cheuvront, S. N., Carter, R. 3rd (2006) Human water and electrolyte balance with physical activity. U: Present Knowledge in Nutrition, (Bowman, B., Russell, R, ured.) Washington, D.C: International Life Sciences Institute.
 Gigou, P. Y., Dion, T., Asselin, A. (2012) Pre-exercise hyperhydration-induced bodyweight gain does not alter prolonged treadmill running time-trial performance in warm ambient conditions. Nutrients. 4, 949-966.
 Cheuvront, Samuel N., Robert Carter III, and Michael N. Sawka. "Fluid balance and endurance exercise performance." Curr Sports Med Rep 2.4 (2003): 202-8.