If we have to ditch our aircraft, be it in a lake or perhaps even the ocean, it is quite possible that swimming to shore will be out of the question. We're not going to be able to keep dry, but there are things we can do to keep warm and stave off the onset of hypothermia. The need to assume the "heat escape lessening posture" (HELP) is critical. This position is assumed by holding your arms tightly to the sides of your chest, crossing your forearms over your chest, and drawing up your legs and crossing them at the ankles. This position closes off most major heat-loss areas. If there are several people involved, huddling close, side to side in a circle, will also help preserve body heat. If you have a hat, put it on, because the head is a major heat-loss area.
If you will be flying beyond gliding distance to land, it is wise to have some kind of life jacket on board, or better yet on body. SOSpenders are a type of life jacket that can be worn while flying, without limiting any body movements. Be sure not to inflate any life jacket until you have evacuated the aircraft.