This is a great thread, like so many on pprune!
A comment on the question of transmission coming unglued - This is a weakness of some designs, but not a pervasive helicopter problem. Most helos keep the gearbox in place when blades contact objects and break off. The exception seems to be the 2 bladed Huey family, whose transmission does unseat when blade strikes occur.
I strongly suggest that a roll will help disorient the occupants, and also make blade-water contact occur at higher rpm (more energy, more damage). Keep the aircraft level as long as possible, pull off the rpm with collective and settle in. Do not release belts until aircraft motion had stopped, a slight head injury makes you an unconscious sinker, not a conscious swimmer.
On the way down before water landing, spot the belts, and even lay your hands on the release. This muscle memory trick is important for performing the act while shockingly cold, blind, underwater and scared. If you rotate the aircraft during water settling, the sharp roll and even upside down aircraft are very disorienting, up is hard to identify, and you are much more likely to go down with your ship.