If rescue is not imminent, your number one priority is to get yourself out of the water as soon as possible. You can do this either by climbing onto your capsized boat or any other floating objects, or if it is within reach by swimming to shore. The sooner you can get your body out of the water the greater your chances of survival will be.
If you manage to make it to shore, be careful not to try and stand up if the current is too strong. Doing so can result in a shallow-water drowning - where a person's feet get caught in rocks or sand on the bottom, and the strong current forces their upper body underwater, causing the person to drown.
If rescue is imminent - you should conserve energy and body heat. You may extend your survival time by wearing your lifejacket.
H.E.L.P. - 'heat escape lessening posture.' You want to keep as much body heat as possible from escaping. If you are alone, cross your arms tightly against the chest and bring your knees up close to the chest.
If other passengers are in the water as well, use the 'Huddle' technique to maintain body heat. Get the sides of everyone's chest close together with arms around the back and legs intertwined.
Additional Cold Water Protection:
Floater suit - a full nose-to-toes PFD
An anti-exposure worksuit - a PFD with a thermal protection rating
A dry suit - used in conjunction with a PFD and a thermal liner
A wet suit - traps and heats water against your body
An immersion suit - used in extreme conditions (usually off-shore)