Swim Practice Hydration: Why You Still Dehydrate in the Pool

Yes, you still dehydrate in the pool. Swimmers sweat during hard training just like land athletes — researchers who measured elite swimmers found real, consistent sweat losses during practice — but the water washes the sweat away the instant it forms, so you never feel it. Add in the fact that cool water blunts your sense of thirst, and swim practice becomes one of the easiest workouts to finish under-hydrated. The fix is simple: keep a filled bottle on the pool deck and drink at every set break, not just when practice ends.

You sweat in the water — the research is clear

It sounds like a contradiction, but sweating is your body's response to rising core temperature, and a hard swim set raises core temperature regardless of the water around you. A study of elite-level swimmers and water polo players published in the Journal of Science and Medicine in Sport measured average sweat rates of roughly 138 ml per kilometre of training in male swimmers and about 107 ml per kilometre in female swimmers (Cox et al.). A separate study of well-trained swimmers in the International Journal of Sport Nutrition and Exercise Metabolism put mean sweat rate around 0.31 litres per hour of training (IJSNEM). Over a long practice with thousands of metres of volume, those losses add up — and that's before you count the fluid you lose simply breathing hard for two hours.

Why you never notice it happening

On land, sweat is your dashboard warning light: a soaked shirt tells you to drink. In the pool, that signal is erased the moment it appears. The water also keeps your skin cool, which dampens the thirst response — the same reason people under-drink on cool, cloudy days. And there's a practical problem: your bottle isn't in your hand between laps the way it is between weight-room sets. Unless drinking is built into the structure of practice, it simply doesn't happen. The research on swimmers bears this out — fluid intake varies enormously from athlete to athlete, and plenty of swimmers drink far less than they lose (Cox et al.).

Build drinking into the practice, not around it

The good news: dehydration in swimmers tends to be milder than in land-based athletes, so modest, consistent intake keeps you ahead of it. Make the wall your trigger. Arrive with a full bottle and place it at the end of your lane. Drink a few good gulps at every set break and between main sets — small amounts often, rather than chugging the whole bottle at the end. If you want to know your personal number, weigh yourself before and after a hard practice: the difference (plus whatever you drank) is what you actually lost, and it will tell you whether your deck-side habit is keeping up.

The deck-side bottle setup

A pool deck is a rough place for a water bottle: chlorinated puddles, hot sun through the windows or open sky, and 15-second drink windows. An insulated stainless bottle solves the temperature problem — water that stays cold for the whole practice is water you'll actually want to drink — and a chug-style lid solves the time problem, because you can flip it open, take three fast gulps one-handed, and push off for the next set. A larger bottle means no mid-practice refill trips while your lane keeps swimming.

The NuRich 32 oz Insulated Water Bottle ($29.99) holds enough for a full practice and keeps it cold on a hot deck, and the NuRich Loop Chug Lid ($9.99) is built for exactly this: fast, high-volume gulps in a short break, with a loop that clips to your mesh gear bag. See the full lineup at the NuRich collection.

The bottom line

Being surrounded by water doesn't hydrate you — swimmers sweat, measurably, every practice, and the pool hides every drop of it. Bring a full, cold bottle to the deck, drink at every wall break, and check your before-and-after weight once in a while to keep your habit honest. You'll finish practice fresher, and tomorrow's practice will thank you for it.

This article is for general informational purposes only and is not medical advice. Individual hydration needs vary; consult a healthcare professional or your coach about your specific needs.

Sources: Journal of Science and Medicine in Sport — Body mass changes and voluntary fluid intakes of elite level water polo players and swimmers (Cox et al.); International Journal of Sport Nutrition and Exercise Metabolism — Water and salt balance of well-trained swimmers in training.