I bought an aviation immersion suit from Andark Marine in Swanick near Southampton for about £200. It makes you look a bit of a prat but combined with the right clothes underneath extends your survival time in water by a couple of hours at the coldest time of year (March) when the sea is around 2-5 degrees celsius. (In a dinghy you can last almost indefinitely)
It's not too uncomfortable flying in it and it behaves as a flight suit as it has lots of pockets etc. I always use it October to April.
I have also been on an underwater Helicopter/Light aircraft excape and survival course at the same place. Its a one day course and it taught me that if you ditch and end up underwater isnide the cabin you will get out if you are alone and following the course instructions. If you have others in the plane who have not experienced the effects underwater (especially upside down) then the chances of getting out rapidly diminish.
http://www.andark.co.uk/main.html
I also carry dinghy, lifejackets, handheld gps, transceiver, dry sweater and beacon all in a waterproof bag.
With family on board it is always the shortest route over sea and thorough briefing (and usually airways!)