Most boating fatalities involve small boats such as canoes, rowboats, and powerboats with less than a 10 horsepower motor. Most boats, even when filled with water, will support the weight of its occupants. If possible, right your boat, re-enter it and then pail the water out. If the boat cannot be righted, climb on top of it. Physical exercise such as swimming or treading causes the body to lose heat at a much faster rate then remaining still in the water.
Blood is pumped to the extremities and quickly cooled.
Few people can swim in 50 degree water. Contrary to popular belief, a couple shots of brandy or whiskey will not warm you up. You may feel warmer since the alcohol opens up the blood vessels close to the skin, but actually, you are greatly increasing your heat loss. Cold water is dangerous enough. You don't have to add the problems associated with alcohol to further decrease your survival time.