Originally Posted by codprawn
One bit of advice they always give you is to move around as little as possible to conserve heat. This goes against the grain for me. I used to do a lot of mountain walking/climbing and the only way to keep warm was to keep moving - stop and you die!!!
You have to bear in mind the differences between loosing heat into the air and loosing heat into the water.
Due to the comparative inefficience of air as a heat-transfer medium, the body can metabolise enough heat to counterbalance the heat lost into the air (I don't know what temperature this becomes untrue). Movement, increases the metabolic rate, and so heat production.
With a human body in water of less than 25C the heat loss into the water already outweighs the metabolic heat production. So by using up energy to move you are not gaining anything - the energy would be better used in maintaining the survival of the core organs.
So, the water in the UK will kill you through cold. It's just a matter of time. It will even kill the channel swimmers etc... although as has already been said, acclimitisation plays a large part in their abilities.