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Pr Peronnet answers you

"The short answer is no, because the swimmer is less at risk than other athletes. The reason for this is that it's much easier to lose the heat produced during exercise to the surrounding water for the swimmer, than to the surrounding air for the runner or the cyclist.

Because of that, actaully the swimmer is much more at risk of becoling hypothermic than hyperthermic during exercise.

So he needs to drink less than, for example, runners or other athletes. So it is a myth to believe that the swimmer needs to drink as much as other athletes."