Of course, the best prevention is to take all measures necessary to avoid capsizing your boat or falling into cold water in the first place. If you do fall into or must enter cold water:

• Don’t panic. Try to get control of your breathing. Hold onto something or stay as still as possible until your breathing settles down. Focus on floating with your head above water until the cold shock response abates.
• When your breathing is under control, perform the most important functions first before you lose dexterity (10–15 minutes after immersion).
• If you were not wearing a PFD when you entered the water, look to see if one is floating around you and put it on immediately. Don’t take your clothes off unless absolutely necessary. A layer of water trapped inside your clothing will help insulate you.
• Focus on locating and getting everyone out of the water quickly before you lose full use of your hands, arms, and legs. Try to reboard your boat, even if it is swamped or capsized, or anything else that is floating. Get as much of your body out of the water as possible. Even though you may feel colder out of the water, the rate of heat loss will be slower than if immersed in water.
• Be prepared at all times to signal rescuers.