The practical answer is yes, but the amount depends on the session
- You still sweat while swimming, but water washes it away before you notice.
- Warm pools, hard interval sets, wetsuits, and long open-water sessions usually increase fluid loss.
- A swimmer can finish dehydrated without feeling sweaty, because thirst and skin dampness are poor guides in the water.
- For most club swimmers, daily fluids plus water on deck are enough; longer or hotter sessions need a deliberate plan.
- Checking pre- and post-swim body weight is the fastest way to estimate how much you lost.
Why you still sweat in the water
Your body does not switch off sweating just because you are submerged. When you swim hard, your muscles produce heat, and your nervous system uses sweat to help manage that heat through the same eccrine glands that work on land. The difference is that you are already wet, so the sweat is harder to notice and easier to dismiss.
I think of sweat as a heat-management system, not a sign that the skin must feel sticky. In a pool, the surrounding water and the air around the deck change how that heat is lost, but they do not remove the need for fluid loss. A swim cap, a wetsuit, or even a long hard set can trap enough warmth to make sweating more relevant than many swimmers expect. Once that is clear, the real question becomes what changes the amount you lose from one session to the next.
What changes how much you sweat in a swim
Not every swim carries the same fluid cost. Temperature, intensity, and how long you stay in the water all change the picture, and the same swimmer can look very different on a cool easy session compared with a hard race set.
| Situation | What usually happens | Practical move |
|---|---|---|
| Easy lane swim in cool water | Heat load stays lower, so sweat may be modest and easy to miss. | Normal daily hydration is often enough. |
| Hard interval set | Muscle work rises quickly, and internal heat climbs even in the pool. | Drink before and after, and keep water on deck. |
| Warm indoor pool or swim camp | Cooling is less efficient, so sweat loss can climb. | Use a bottle regularly and consider sodium for longer sessions. |
| Open water with a wetsuit | Trapped heat can make you sweat more than you expect. | Hydrate before the session and replace fluid afterwards. |
| Long steady training | Small losses build up over time, especially when you do not sip. | Track body weight if the session runs past an hour. |
The pattern is not random. Session intensity, water temperature, air temperature on deck, body size, and how well you are acclimatised all matter. That is why two swimmers in the same lane can finish with very different fluid needs, which leads directly to the dehydration question.
Why dehydration can still happen in the pool or open water
Fluid loss does not stop just because you are surrounded by water. You still lose fluid through sweat, and you also lose some through breathing. The problem is that swimmers often miss the early clues because skin wetness tells you almost nothing, and focus on pace makes it easy to ignore a dry mouth or a creeping headache.
When I look for dehydration in swimmers, I watch the session result before I watch the mirror. Common signs include:
- your pace fades while effort feels the same
- you get a headache or feel flat after the session
- your mouth is dry once you get out
- your urine is darker than usual later in the day
- you feel light-headed when standing up on deck
A useful checkpoint for longer sessions is body weight. If you regularly finish more than about 2% down, I would assume you are under-replacing fluid rather than just having a hard day. That is why the next step is a simple drinking routine before, during, and after the session.
How to hydrate before, during and after a swim
For most swimmers, hydration works best when it is boring and repeatable. The goal is not to chug huge amounts at once, but to arrive already prepared and then replace what you lose in a steady way.
Before
On a normal day, the NHS guidance of 6 to 8 cups or glasses of fluid is a sensible baseline, with more if you are active or in a hot environment. Before a longer swim, I usually like an adult to have roughly 300 to 500 ml of fluid a couple of hours beforehand, then a small top-up if the urine looks darker than pale straw. That is enough to start the session ready, without sloshing around in the lane.
During
For sessions under 45 to 60 minutes, water between sets is often enough. Once you go longer, take small sips at every break rather than trying to catch up at the end. A simple starting point for longer swims is 150 to 250 ml every 20 to 30 minutes, adjusted for how much you actually sweat and how easy it is to drink on deck. For hard or long swims, a drink with sodium makes more sense than plain water alone, especially in warmer pools or open-water events.
Read Also: Why Swimmers Pee So Much - Hydration & Immersion Diuresis
After
Weigh yourself before and after longer swims if you want a clearer picture. Each kilogram lost is roughly a litre of fluid, and replacing about 1.25 to 1.5 litres for every kilogram lost over the next few hours is a practical target. Pair that with a normal meal or a salty snack so the fluid stays where you need it, rather than vanishing straight back out again.
Once that routine is in place, it becomes much easier to separate fact from the usual myths swimmers repeat about sweat and water.
The myths that keep swimmers under-hydrated
- “I did not feel sweaty, so I did not lose much.” Not true. In the pool, sweat disappears fast and your skin never gives you a reliable readout.
- “Cold water means no sweat.” Also not true. Cooler water may reduce sweating, but hard work can still create a meaningful fluid loss.
- “Only runners need hydration planning.” False. Swimmers lose fluid too, and in long sessions the loss can matter just as much for performance.
- “Sweat is just water.” Not quite. It carries sodium and other electrolytes, which is why very long or hot sessions deserve more than plain water alone.
- “If I am not thirsty, I am fine.” Thirst lags behind reality, especially when you are focused on pace, turns, or open-water navigation.
My view is simple: the myth that causes the most trouble is the one that says swimmers do not need to think about hydration at all. That is also why unusual sweat patterns should be taken seriously rather than shrugged off.
When sweating patterns deserve attention
Some variation is normal. A bigger swimmer may produce more heat, a hard set may trigger more sweat than an easy one, and a wetsuit can change how warm you feel. But if your sweat pattern looks extreme or oddly absent, it is worth paying attention.
I would speak to a GP if you repeatedly sweat heavily in cool water, hardly sweat at all during hard exercise, or notice dizziness, chest pain, palpitations, faintness, or persistent headaches around training. Medicines, hormones, thyroid problems, and conditions such as hyperhidrosis or anhidrosis can all affect sweat patterns, and that is not something to solve with another bottle of water. In other words, when the pattern looks wrong rather than merely annoying, treat it as a health issue.
The routine I would use for most swimmers
- Arrive already hydrated instead of trying to catch up on the poolside.
- Keep a bottle within reach and sip during breaks on longer sessions.
- Use before-and-after body weight to judge fluid loss on harder or longer training days.
- Treat warm pools, wetsuits, and interval sets as higher-risk sessions for dehydration.
For most recreational and club swimmers, that is enough. The practical answer is not to obsess over sweat, but to respect it: you do lose fluid in the water, and your body gives you weaker signals there than it does on land. If you keep the routine simple, consistent, and matched to the session, you will cover most of the problem before it affects performance.
