Fluid replacement is just as important in swimming as it is in any other athletic activity. You may not think you sweat a great deal when you swim because the water quickly dissipates any excess heat your body generates, but in reality you do sweat, losing fluids in the process. In fact, swimmers lose approximately 120 ml of fluid per 1000 m of training at a moderate pace. This amount of fluid can be increased up to 160 ml per 1000 m when the training intensity increases. Swimmers who train for at least 5000 m per training session may lose up to 800 ml of fluid. All this fluid should be replaced before the next workout. While this makes sense, swimmers typically will replace only 30 to 70 % of the fluid lost during a training session when left to their own devices.
