Surviving in Cold Water
When traveling across the oceans it's easy to forget that the water is likely to be quite cold even when the weather is warm. In fact, water temperatures below 70 degrees can cause hypothermia. Crazy, isn't it? So in a catastrophe where you find yourself floating in the water, your primary objective will be to avoid succumbing to the cold long enough so that you can be rescued.
First off, do not attempt to swim unless it is for a very short distance. A general rule of thumb to remember is that a strong swimmer has a 50-50 chance of surviving a 50-yard swim in 50-degree water. Those odds are bad so be sure to heed them. The only reason to swim is if safety is close by or if there's an object that will make it easier for you to remain afloat. Try to remain still to keep your heart rate low. And of course, breathe normally.
If you are alone and wearing a flotation device, there's a position you should take to reduce your heat loss by up to 50%. Cross your ankles, draw your knees to your chest, and cross your arms over your chest. Keep your hands high on your chest or neck to keep them warm. Do not remove your clothes as they will not weigh you down, but they will provide some warmth by holding warm water close to your skin.
If there are other people in the water with you and everyone is wearing a flotation device, gather together and "hug" with chests touching chests. The closeness will help you share heat and also has the side benefit of making you more visible to rescuers.
If you aren't wearing a life jacket, the same rules apply. However, seek out debris to help you float. If nothing is available, you can float on your back or tread water slowly. As a last resort, you can attempt to fill your jacket with air from the bottom. Be warned that this sort of movement can hasten the cooling of your body.
The Worst Case Scenario Survival Handbook - Travel by Joshua Piven and David Borgenicht