Why does water feel cold when air would feel warm?
Why does 27 degree air feel hot but water of the same temperature feels cold?
That's to do with conduction. Water conducts heat much more readily than air so it's simply sucking heat out of your body, which makes you feel cold. If you think about wind chill, you can go out in the Antarctic and on a very still day which is minus 30 degrees you won't actually feel that cold. It's the movement of the wind past you that makes you cold. Those air molecules passing over you are each stealing a little bit of energy from the surface of your body and then moving on so you cool down much quicker. If you're wearing a wetsuit in water you have a single layer of water around you that warms up. Without a wetsuit the water is moving around you and robbing your body of energy all the time and cooling you down. This is why if you fall into cold water you should keep your clothes on and stay still and not swim around.