Why does your body temperature go down when you swim?

Water conducts heat better than the air

Heat flows out of your body through three mechanisms: conduction, convection and radiation. Conduction occurs when heat directly flows out of your body when it comes into contact with a cooler object. Convection occurs when cooler air or fluid moves over your body, taking the heat with it. Radiation is when your body emits infrared radiation, which can be transferred to anything in the area.

The air is not very good at conducting heat. This is why it doesn't feel so cold on your skin when you step outside on a cold day, as long as there's no wind. But water on the other hand is very good at conducting heat. Because water is denser than air, there are a lot more water molecules that can come into contact with your skin, carrying all that heat away. In fact, water is about twenty times more effective at transferring heat than air. As soon as you step into a swimming pool, you start losing body heat through this conduction process, which is why it often feels "refreshing".

Evaporation cools your body

As water gets warmer it evaporates into water vapor. This process of evaporation also draws body heat away from the skin, effectively using it to turn liquid water into water vapor. Water vapor then floats off into the surrounding air. If the air around you is also water vapor saturated (this is more likely on hot days or in humid climates), it can't take on more water vapor. When this happens, this means you can't cool yourself efficiently by sweating, which is the reason why it feels so hot.

Air Temperature

Of course, the temperature of the air and the water also play a role in how cold you feel. If the water temperature is close to your body temperature (around 37 degrees Celsius), you shouldn't feel too cold. However, if the water is a significantly lower temperature, such as if you were to jump in a cold mountain stream, then your body temperature will quickly drop.

Acclimation

Finally, your body has a knack of getting used to things, including colder water temperatures. If you swim regularly in water between 20 and 25 degrees Celsius, you will find that after a couple of weeks, your body will be able to conserve more heat and you won't feel so cold.