system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does 90 degree heat feel hot while a 90 degree swimming pool feels amazing?
gpt
Water is way better at transferring heat than air is.  First off, it's got a higher heat capacity (It takes 4.2 KJ to warm a kilogram of water by 1 degree, whereas for air, it takes about 1 KJ to do the same). It's also a more effective thermal conductor - water has a thermal conductivity of 0.6 W/mK, air has a thermal conductivity of .025 W/mK.

In layman's terms, it takes more energy to heat up the same mass of water, and it's easier to give up your heat to water than it is to air (because it's a better conductor).  Your body has a hard time giving off its heat to 90 degree air, but it can easily give off its heat to 90 degree water.

The other thing that makes it hard to cool yourself on a hot day is humidity.  Your body cools itself in air by sweating.  If it's humid, there's nowhere for your sweat to go - instead of evaporating, it stays on your skin, because the air is too saturated for your sweat to evaporate.  In water, you don't need to worry about the fact that your sweat isn't going anywhere, because the cool water will gladly take the heat from your sweat and from your warm skin.