While you may not want to go swimming in frigid waters, you do not always have the luxury of making such choices. Sometimes unforeseen things happen and you are left to deal with the aftermath. This could mean any of number of things going wrong and dumping you in cold water, such as a makeshift raft that sinks or an ill-timed step on the water's edge that results in a slip that causes you to go for a dip in water of which you are unable to climb out. Preventing such accidents is ideal but you cannot always see danger coming until it grabs ahold of you and pulls you under the surface.
The most frequent location for hypothermia to occur is in the water because your body loses heat in water 30 times faster than it does in air. This does not mean you cannot get hypothermia on land, just that you will get it much more easily in water. Hypothermia occurs when your core body temperature dips below 35C or 95F. Upon reaching such low temperatures you will experience a decrease in awareness and an inability to think, move, or function normally. Your body will start to make sacrifices, opting to preserve internal organs by restricting blood flow to extremities. If your body cools down to 30C or 86F, heart failure becomes more probable than possible.
To survive hypothermia, there are a few steps you can take. First of all, do not panic. This sounds easier said than done, but in truth, allowing yourself to panic will cause your heart rate to elevate and makes you breathe faster which burns more heat and energy. It is important to maintain steady breathing, especially if you are diving with a regulator as you will rapidly use up your air. If you are trapped underwater and run out of air, hypothermia quickly becomes a lesser problem.
When you find yourself stranded in cold waters, the first thought that may come to mind is to swim. This is not wise in hypothermic conditions, however, as swimming leads to faster loss of body heat. Muscles are also more prone to cramping in cold water, making them unresponsive and ineffective. Optimal swimming capabilities of a strong swimmer in cold waters would only give you about a half mile swim, and that is in calm waters. If you are further from shore than that or have a current to fight against, do not risk it.
It may make sense to remove shoes and clothing so you can float above the surface with greater ease, but those items have now become necessary for your survival. Anything on your body now acts as insulation, helping you to retain body heat. If you have a life jacket, do not remove that item either, as it will help keep you afloat in addition to trapping heat. Remember that treading water will also lead to fast loss of heat, so keep that flotation device on at all times and do not rely on your own movements to stay afloat if you can avoid it.
Another way to stay above water is to cling to debris in the water. If a cooler or other floating debris is nearby, holding onto it can keep you floating. Climbing onto the hull of an overturned boat can also be a useful tactic and gives you a higher profile in the water. Plus, if you are in an ocean-going vessel equipped with an EPIRB (Emergency Position Indicating Radio Beacon) and the Coast Guard is still active, they will be better able to find you if you remain near your boat's wreckage.
If you are alone in the water with nothing to hold onto, then hold onto yourself. Bring your feet as close to your body and wrap your arms across your chest. Make yourself as compact of a heat ball as possible while still managing to keep your head above water. Should you be with others, huddle together to share warmth.
Hypothermia can cause delirium, but try to remember to stay positive. Watch for others coming near enough to see you and signal them that you are in distress. If you have anything metallic or reflective, use it to reflect sunlight at other boaters in the area. Use a piece of brightly colored cloth as a flag and wave it if a potential rescuer comes into view. Keeping a high profile in the water will better allow you to be seen. Remember that while hypothermia can be difficult to defeat, it does not have to defeat you. Use good sense while in and around water or other extremely cold conditions; carry a whistle, strobe, and other safety gear. Always prepare for the worst, but never lose faith that the best is yet to come.