Hypothermia occurs when you capsize in a kayak, get wet and you are not wearing the correct clothes based on the water temperature. You can't get back into your kayak fast enough and your body core temperature starting falling. As your core temperature continues to fall, you become confused, you lose control of your hands and possibly heart failure occurs. It is easy to learn how to prevent hypothermia.
Help and huddle positions
The help position in the water is assumed be placing your arms close to sides of your chest with your legs crossed in back and bent up to close off the groin area from the water. The huddle position is used when there are more than one person in the water. By having people stay together, it slows down the process of losing body core temperature. Everyone stays very close to one another and remaining as still as possible, trying to keep as much of the cold water from touching parts of the body.
Stay as calm as you can be in the circumstance and try to get back into the kayak ASAP. Don't waste any unnecessary motion to get back into your kayak. If you can't get back into the kayak, at least try to get yourself on to the overturned kayak so that you are at least raised out of the water.
If you aren't very close to shore, forget swimming. Swimming will increase heat loss, cramps and hypothermia will likely develop more rapidly. Stay next to your boat. Consider all the options before starting to swim.
Always dress for the water temperature, not the air temperature. The shock of the cold water on your head when it hits the water will be dramatic. Always dress appropriately for kayaking: no jeans, shorts, cotton shirts, sneakers, etc. Always wear clothes designed for kayaking and ALWAYS wear a PFD. There should be an extra set of clothes stored in a dry bag in the hatch.
Always paddle with a partner and never paddle alone. Be sure you know self rescue as well as assisted rescues since there might be a situation that both of you might be in the water at the same time.
Get to land
If you are able to get back into your kayak, proceed to the nearest land where you can change into the extra set of dry clothes stored in the hatch.
Proper prevention in advance will minimize the risk of hypothermia. Hypothermia can even occur on a sunny 70 degree day. Learn how to prevent hypothermia.