In as much as most of the globe is covered by salt water, odds are that a successful "water landing" will deposit the passengers near water in which floating will be somewhat easy, vest or no vest, even for nonswimmers. Problem will be getting out of the airplane into that open water.

A freshwater landing would make the vests all that more important, of course.

And if the airplane breaks apart on "landing," making access to the open water very easy, odds are everyone will already be floating (or at least pieces of them will be floating), vest or no vest.

I hope no 777 or 747 ever has to land in the middle of the ocean and prove me wrong (by gently touching down like a seaplane). Much better to land on the runway.