It's common sense.. I was a frequent flyer as a child and I always understood.

Here was my thought process and plans as a child in case of a water crash, the jacket is going to do one of several things a, inflate to keep you afloat for enough time so that the rescuers can get to you and keep you from drowning, b, assuming the emergency exits are open and the pressure from the water doesn't trap me in a giant hunk of aluminum waiting for my impending doom, wait until the plane has filled up, escape and inflate or escape and inflate after exiting (underwater), and c, assuming the plane has sunk and you were unable to escape before being too deep allowing for air pockets to supply your oxygen until you are overcome by water, the vest would inflate and bring you to the surface many times faster than by physical force if you were to escape.

Of course there are several factors which contribute to that, one being, if we were deep enough and I was able to exit after using up the oxygen from the pockets of air inside the cabin, I would need to breathe in and at the same time release an equal amount of air at a steady speed to insure I don't get fucked up from the sudden change of pressure which could cause me to black out, collapse and or burst a lung or two.

Going back to point a, the life vest has a supply of air that keeps it inflated for a certain amount of time, at the same time air is slowly escaping so over a period of time your vest will deflate (assuming it doesn't have blow tubes to reinflate or it isn't damaged) so if you're lucky enough to be in a body of water that doesn't have predators, which by the way the elements would most likely kill you faster than a shark or jelly fish (hypothermia, thirst etc.)

Using the rule of three, let's determined how fucked you are likely to be if you land in a remote part of the ocean, 3 days no water, 3 weeks no food, 3 hours no shelter, 3 minutes without oxygen.

In a crash, the plane will stop sending out signals, that being said there will probably be a search team assembled and they will be dispatched to the LKC. That being said, keep in mind the rule of three, assuming you are in a remote location and they are taking a helicopter if the coast guard doesn't have a boat nearby, it could take hours, by then you will have drifted several miles away from the crash sight due to the current, the plane will almost be completely submerged and you will be suffering from early signs of hypothermia due to your core temp being reduced.

Obviously there are rare exceptions to the rule, people surfing months out at sea, but let's be realistic. More than 80% are probably gonna be dead by the time the coast guard even gets to the last know coordinates.

What I would do is this, after we hit, assuming I'm still conscious (lol) I would put the vest on, by then many people will be screaming like idiots while I'm trying to think, the exits will inflate and I will look for assets to keep my ass alive. Many people, will ignorantly jump in the water starting the countdown to hypothermia much faster than everyone else, don't jump in the water until there is no more surface to keep you separated (assuming the plane is not in danger or currently combusting, in that case, go on the life raft with everyone else, oh and follow your airlines safety procedures and policies, not somebody on reddit) but what I would do is climb on the wing and or on top of the aircraft with my assets gathered from suitcases, I would then proceed to sit down, watch chaos ensue and think. Once I determine the best course of action assuming the coast guard isn't there yet, I will then help people get to safety and calm them down.

Well that didn't make any sense, but it's a cool brainstorm I guess.