The decision to put the personal flotation devices on board was made by people a lot smarter than the people who think they are for use after hitting the water at 400 knots.
Of course they are for controlled ditching. The only controlled ditching of a jet airliner that I am aware of was an ONA
DC-9 back in the 1970's off St. Thomas in bad weather. I don't have immediate access to any of the details but I think that I recall that everyone survived the ditching and that most or all of the fatalities were caused by the inadvertent inflating of the 46-man raft in the forward galley area, blocking the 1L and 1R doors.
There is also the famous ditching of the PanAm B-377 at Ocean Station November, between California and Hawaii in about 1956. There were passengers on that flight who never even got their shoes wet.
Extended overwater airplanes will have 46-man rafts, enough to hold all occupants of the aircraft plus something like a 50% overflow capacity. The life vests will give you a much better chance of making it to the raft.
To be sure, the eastbound night crossings of the north Atlantic carry a higher risk than, say, a daylight flight to Hawaii. I asked the Hibernia drilling platform about their weather and sea state one night. They replied that the seas were fairly calm with 1.5 meter swells. Not good odds on ditching but some would probably survive it. The water is cold. You would not last very long in it, and it will begin to rob you of your strength almost immediately. Again, those vests are just to get you to the raft. Some would survive.
I'd bet that those who would survive were those who worked at it - those who did not just apathetically believe that if anything goes wrong they are going to die. We cannot do anything about the wildcard luck
items, but the rest of it our will to live can make the difference.
Most of our overwater air routes approximate the great circle routes. That means that if there are ports at each end of the route there are ships navigating them too. Ships mean rescue within hours now. With the AMVERS program the pilot may even be overheard by ships when making the "mayday" call and may even talk to a ship on the way down. It is not completely hopeless and I personally would rather die busy than feeling sorry for myself.
If all the life vest would do is keep my body afloat then I owe at least that to those left behind. My family could at least have closure if my body is found. I'd put it on for that reason alone.
Happiness is not seeing another trite Ste. Maarten photo all week long.