Well, honestly they should provide both in case of emergency evacuations although I think they do, now. (Don't hold me to that because I'm not really sure).
How ever, even a Buoy really doesn't make sense because usually at the point of impact in the Ocean the Airplane is destroyed by the pure force of the impact which kills most if not all of the crew and passengers instantly.
So..
Really, if you think about it, with all of the pushing and shoving while the panic-ridden attempts to evacuate, both ways are sort of rendered useless.