Why Does Water Feel So Cold?
A 15-degree day is cold but not extreme, but water of the same temperature is almost unbearable. It's because water’s ability to conduct heat is greater than that of air, “stealing” heat faster from the body. The opposite is also true, if the temperature is higher than that of the body.
Materials' heat conductivity depends very much on temperature and on the type of material. Many metals' conductivity is high, as electrons are free to transfer the heat rapidly. In other solids, the heat is primarily transferred by atomic lattice oscillation.
Water “steals” heat more quickly from your body than air does.