Hydration (or the amount of fluid taken in by the human body) is just as important as food intake before and after exercise. Two hours before exercise, swimmers should consume 16 fluid ounces (or half a litre) of water or a sports drink to help hydrate them ahead of time. Thirty minutes before exercise, athletes should intake another 8 fluid ounces (quarter of a litre) to prepare themselves for activity.

During activity, fluids should be available for swimmers at all times. Because swimmers are sweating out important fluids, they must replenish them by drinking 8 fluid ounces (quarter of a litre) every 20 minutes. Swimmers should always bring a plastic drinks bottle with them to training sessions. Long-term, moderate to intense activity of 30 minutes or more requires periodic re-hydration, such as the 8 fluid ounces (quarter of a litre) every 20 minutes just suggested. If an activity lasts more than 40 minutes, water is not sufficient to re-hydrate the body. The nutrient loss through sweat requires a sports drink to replenish electrolytes (powder energy drink is available at most sports outlets).

Many swimmers will prefer not to drink during activity or will feel ill directly after intense exercise. All athletes must drink adequate liquids before, during, and after activity to avoid dehydration, which can lead to nausea, dizziness, and fatigue. After activity, swimmers should continue to intake fluids. At this point, fluids can be the normal amount the athlete would consume with a meal and through the rest of the day. A total of 64 fluid ounces (1.8 litres) of fluid is a minimum for swimmers, though more is recommended throughout the day.

A good test of proper hydration is a urine test. Swimmers should pass clear urine, not dark or with a restricted flow. Swimmers should pay attention to their own needs, as all swimmers will have slightly different needs. If a swimmer feels uncomfortable, light-headed, or feeling unwell, they should bring this to the attention of their coach. As fluid intake levels will change based on environmental effects, swimmers should pay attention to the outside influences affecting their fluid needs eg temperature, use of regular medication, intake of certain food substances before training/competition, can all affect the hydration levels of a swimmer.