Not drinking sufficient water will lead to dehydration, especially in endurance sports.
If you search for dehydration and sports you will find a lot of references, I just give one example which also describes why dehydration is a problem in endurance sports.
MURRAY, R. Nutrition for the marathon and other endurance sports: environmental stress and dehydration. Med. Sci. Sports Exerc., Vol. 24, No. 9 Supplement, pp. S319-S323, 1992.
- During exercise, the body's ability to safely regulate internal
temperature is influenced by the environment, exercise intensity,
clothing, and the athlete's level of fitness and acclimation.
- Effective thermoregulation during exercise in the heat requires
the evaporation of sweat. The onset of sweating during exercise is
triggered by an increase in core body temperature.
- Dehydration compromises cardiovascular and thermoregulatory
function, limits physical work capacity, and increases the risk of
heat-related health problems.
- Physiological and behavioral adaptations help the body cope with
the combined demands imposed by exercise and environment.
- Ad libitum fluid intake is insufficient to protect against
It is clear that dehydration leads to worse prestations. See eg this synthetic test:
Therefore, it appears that competitive performance in trials of long
duration (5,000 and 10,000 m) was affected to a greater extent by D
than the shorter 1,500-m event, even though submaximal and maximal
oxygen uptake was not altered.
Drinking water during an exercise can make sure that you remain hydrated  or get rehydrated both improving the performance.
For intense prolonged exercise lasting longer than 1 h, athletes should consume between 30 and 60 g/h and drink between 600 and 1200 mL/h of a solution containing carbohydrate and Na+ (0.5 to 0.7 g/L of fluid). Maintaining proper hydration before, during, and after training and
competition will help reduce fluid loss, maintain performance, lower
submaximal exercise heart rate, maintain plasma volume, and reduce
heat stress, heat exhaustion, and possibly heat stroke.
Our results demonstrated that the swift replacement of the fluid loss
in the dehydrated subjects was beneficial to exercise performance by
rapidly correcting the disturbances in body fluid balance.