I haven't looked into the legal background, but my guess is that offshore workers' unions have lobbied for it - and, most likely, it only applies to flights to/from sites operated by major players.
Also, some countries refuse to recognize training from other countries (Brazil, I am looking at you!) - so you end up having to do the same basic training in [n] jurisdictions, having [n] different certificates on you at all times, all with their individual refresher requirements...
Depending on where you're embarking from, equipment like this may already be available - though not with a 15-minute supply, more like a minute, perhaps.
When I've flown from Norway in latter years, our survival suits (worn throughout the flight) have been equipped with a rebreather device - you bite down on a mouthpiece connected to a plastic bag integrated in your suit, and as you exhale into the plastic bag, you can then draw the same air back in again - no CO2 scrubbing. The mouthpiece is equipped with a valve which lets you breathe normally while above water; you then close it as you are submerged and breathe into the bag until you've reached the surface again.
The idea being that this will do for a minute or two - and in that minute, you're either out of the wreck or dead.
(I've asked the simulator instructor how long we spend underwater - he shrugged and said 'On average, six to fifteen seconds' - so a minute is a long, long time.)
It is difficult enough in the simulator - which, as you say, simplify matters immensely over an actual crash:
a) The pool with the simulator is well lit. Many places, it is even heated, so you don't get the added stress of being lowered into icy cold water.
b) Each participant has a dedicated rescue diver sitting opposite him, ready to assist at any sign of trouble.
c) The simulator is lowered into the water in a controlled fashion, without any jolts.
d) There's only one seat by each window, so you don't have to first help your mate out, then exit yourself.
Incidentally, I've found it is much easier to get out after the cabin is inverted - your buoyancy makes you pop from your seat like a cork the moment you open the harness; being inverted, you are being pushed towards your seat, and all you need to do is to pop out the window, grab the opening with one hand, undo the buckle with the other and pull yourself out. Much simpler when your buoyancy works with you than against you.
During my last refresher, we asked whether we could make it a bit more realistic by putting out the lights and leaving a few newspapers and such in the cabin (for floating around once it filled with water, further disorienting you.)
We were told in no uncertain terms that wouldn't happen; it was deemed too risky for the rescue divers - who were fully kitted out with oxygen an'all.
Note to self: I do not want to be in a helicopter emergency landing on water. Ever.