As the guy who does research for the guys who will have to pick you up if you ditch.
1. Get a beacon. Without a beacon we probably won't find you and you will die.
2. Wear an immersion suit. Without a suit the initial shock of immersion may well kill you. If that doesn't get you, for most of the year you will die of hypothermia in the hour or so it takes us to get airborne and reach you.
3. Wear a Lifejacket. Without it you will work hard to stay afloat, and that will use up the energy reserves that would otherwise keep you warm.
4. Do an underwater escape course. There have been numerous ditchings where the living and the dead have divided neatly between those who had done the course and those who hadn't.
5. If at all possible take a dinghy. They are available as wearable backpacks replacing your seat cushion. A dinghy increases your survival time from hours to days, and is about fifty times easier to see than your head bobbing around in the water.
Sven