Criticalmass has it right for all the open ocean/water stuff I can recall. I don't believe ther has been but 1 commercial airliner jet ditching ever and a quick search of all NTSB reports from 1996 on doesn't show any.
ON THE OTHER HAND I do recall several rejected takeoff/ landing long accidents at LGA where the aircraft ended up all or partly in Jamaica Bay where the cushions/ lifejackets came in very handy. ther was an accident a number of years ago where a Boeing jet of some kind (I think 727) in Pensacola FL ended up in Escambia Bay and everybody (or just about everybody) lived to tell about it.
Bottom line appears to be on the basis or a very small (thank goodness) data base open ocean type ditching from altitude odds are against, may not make much difference whether you have floatation or not. In the off the runway into the bay, river, etc situation the floataion devices can save your a** big time. Nobody who ever saw the video of the rescue after the Air Florida 737 crash at DCA will ever go anywhere without life jackets or equal.