Much respect to those that have survived a ditching. As OFO says, it's OK pontificating about what to do but the reality is going to be very different. That's not to say you shouldn't prepare obviously.
As an aside though, as a kid (therefore not covered in as much fat as I am now...) we use to swim in the North Sea up the Whitby area all day on and off from April onwards. How come we never suffered from anything cold related? Is the water warmer nearer the shore?