Word I heard is that for flights over water the ramps as rafts, and any rafts, are collectively rated to nominally exceed the passenger capacity of the aircraft. In other words there is, at least theoretically, enough spots on the rafts for all the passengers and aircrew.

Nominal ratings for persons are not very generous. Something like three square feet per person. Which means that if everything goes perfectly, plane intact on a smooth sea, all the slides inflate and are accessible, and the passengers are properly dispersed between rafts that everyone could be part of our happy little navy. Failing that they have life preservers and seat cushions.

This kicker here is that the odds of an airliner making a successful water landing aren't that good even in perfect conditions. Pilots don't practice it and manufacturers don't test their planes by landing them in the water. In part because it usually destroys the airframe even if it stays in one piece. The forces are too high and there are just too many variables to predict.

Pilots are, from what I heard, given only general advice for how to do it. Pretty common sense stuff like draining off speed and planning the approach to end in a nose high, wings level attitude with the plane set up to perform as close an approximation of zero-speed stall a few feet off the water as possible.

One bit of advice offered is to try to land near boats to allow swifter rescue of the passengers. The pilot didn't come down near the ferry piers out of chance. He aimed to be close to the ferry terminals. Good thinking.

As a matter of fact I heard one of the ferry crewmen saying that they practice water recovery, including the deployment of ladders and/or nets less than a week ago. Evidently they train regularly. Makes sense in that people sometimes fall off ferries and they sometimes sink. Ferries crews rescuing each other is good, and if the odd airliner wants to get in on the act, so much the better.

Of course if there is any significant wave action, cross winds, limited visibility, a successful water landing goes from being possible to not so much. Such conditions also pretty much make deployment of slides as rafts and an organized evacuation onto them less likely. Imagine landing in a 60 mph gusting and shifting wind, 20 foot waves of 35F water and visibility of ten feet. Imagine having received a miracle and come down in one piece trying to deploy the slides and keep everyone together. Nobody expects it to go well.

It isn't really much of a problem considering that the odds are the plane cracks up after hitting a wave and all the occupants are stunned on impact and drown. Those few that don't are unconscious from hypothermia in a few minutes and a short bit after that they are dead.

If an airliner goes down in a storm over cold water the odds are your all dead. Good thing, for the most part, they don't go down. There are cases of military pilots going down in storms where they made it but, husky military aircraft, warm water, survival suits, dedicated life rafts, survival training, and a relatively low numbers of people all played a part in making it possible. Even with all that going for them most don't make it.