With regard to high level floats, I was involved in some R & D work for such a system, the overall theory being that the high level floats would trigger on one side only (Higher level Saline switch maybe?) thus preventing total capsize and allowing escape routes to open air. The technology is very simple, however key challenges were flot bag deformation around doors and rotors leading to swift puncture and also where to actually stow the bags. There isn't a lot of room up around the gearbox to house the size of bags you'd actually need for the system to be effective.
The other solution is to mount the main flot gear higher abover the water line and go for a partial sinking of the cabin as a trade off for stability, this technique was suggested for the Apache, however egress from a partially submerged cockpit was an issue.. I think the use of a canopy MDC was mooted.?
As I recall the high level floats project was shelved in 2009 due to lack of interest from CAA / Operators / Manufacturers. I hasten to add that was only from a GKN perspective. I do know others such as Dart and AeroSekur also looked at the idea, but not sure they got much traction either. I'm no longer in the Emer Flot Gear business so my intel may not be current.