Hi
The UK military experience with helicopter uncontrolled ditching is that nearly everyone who has done HUET training has lived and nearly everyone who hasn't has died.
HUET is (in my bit of the UK mil) mandatory for overwater flight at night, mandatory for all crew, and desirable for all overwater flight. Additionally, we are close to insisting on air supplies being worn for all overwater heli flights (the equipment for passengers is still under trial / certification).
I would have thought that the same rules would be appropriate to a flying boat or floatplane.
Sven