I flew with my wife and two friends to France many years ago in a C172. After the experience my wife made me promise not to fly over water again in a single, as our daughter had just been born.
Years later a pilot (who later featured in the TV series '999') ditched a turbine helicopter in the Irish Sea off Blackpool. He was wearing an immersion suit, but only just survived thanks to a passing aircraft dropping him a dinghy. His immersion suit leaked and he recalled how quickly he lost feeling due to the cold.
I think the issues to consider are:
The 'shock' of the experience and the fact that, rather than being a small boy swimming in the sea off Whitby, you will be suddenly cold, in shock and possibly injured;
In an environment which is alien to you, with the added dangers of sea state and sea temperature; and
Unlikely to be 100% prepared in terms of survival equipment, clothing and knowledge of its use - particularly in the adverse conditions of a ditching (I've never seen an immersion suit nor seen anyone wearing one in reality).
I'm sure I read an article in Pilot magazine years ago written by a former Air/Sea Rescue pilot who stated that in all the years he had been flying in that role he never brought anyone out of the sea alive. Some survived the ditching, but of those none survived the sea and the cold.
FOK