According to the old chestnut, "This plane floats as well as a boat flies"...
ChrisVJ, I'm with you up to the emergency exit bit, but enough of my sanctimonious talk, eh AC?
Lifejackets: Granted, even in a survivable water landing, you'd probably die of cold before you'd drown. I suppose the paranoid could take immersion suits.
Safety briefings: Problem is, if you put too much in people really nod off. In my North Sea days, we'd to sit through a 13 min video before every flight. Important information but it never seemed to hit home for everyone.
Incidentally, if a 737 (say) tried a water landing, what would happen when the engines touched the drink? Would the acft pitch down/flip, or would they just shear off?