Try to get yourself up and out of the water, it will help you to keep warm. Cold water will deplete body temperature faster than the air. So try and pull yourself out of the water by climbing onto the boat, dock or a floating log. If you are alone and are unable to get out of the water, wearing a lifejacket, try to slow the heat loss by taking a position called HELP (Heat Escape Lessening Position). This position can greatly improve your chances of survival. Knees up to your chest, arm wrapped around to hold in place and remain calm. Trying to swim will use up energy needed to survive. On the lake you probably would attempt to swim to shore because the chances of making it are far greater. If you are with other people, all in the same situation, try and warm each other up with bodyheat and huddle. This would also increase the groups chances of survival.