If you fall in - and rescue is not imminent - you could extend your survival time by learning the following things:
Depending on your size and physical strength, you may only retain the necessary motor skills to swim for between 10 to 30 minutes. In most cold water immersions you will drown - not as a result of hypthermia or exposure - but from swimming failure!
The initial shock of immersion puts a severe strain on the body and mind - and can induce instant cardiac arrest. Expect to have the breath driven from you upon impact.
If your face is in the water during that first gasp for air - you might get water instead. Disorientation follows shortly after immersion.
You will quickly lose your body heat and the ability to function. Hypothermia will set in. Without rescue and proper first aid the inevitable conclusion is unconsciousness and death by drowning.
You lose body heat 32 times faster in cold water than in cold air. Once in the water, all your strength and effort MUST go towards getting out of the water.
Most accidents involve small boats that - with practice - can be righted and re-boarded. If you are not alone, huddle together or in a group facing each other to maintain body heat.
DON'T PANIC. Let your life jacket work for you. Swimming or treading water can shorten survival time by more than 50%.