I understand that every bit of weight is precious in an aircraft however I am somewhat surprised that no one has put forward realistic solutions to the problems of ditching and remaining afloat.
For an aircraft with a weight of about 1200lbs it takes about 15lbs of polystyrene foam. (I'll expalin hpow I know in a sec.) By choice perhaps polystyrene is not what one would choose, perhaps not the best in a fire and that is the other major danger, however I am sure if one was to pay a little more one could find something even less dense and perhaps less flammable.
How about filling non occupied space with ping pong balls? Maybe inflatable attachable buoyancy.
About the time I was building jemima, a Searey amphibian, our group lost several aircraft to landing in water with wheels down and while most of them were recovered they all sank far enough to douse the instruments and the engine. Most also suffered a collapsing foredeck. I filled Jemima with 15lbs of polystyrene using it also to reinforce the nose and deck. If I wreck her and survive the impact I should be able to sit on top!
Incidentally but connected:
I saw an demonstration on utube of an amphibian landing on water wheels down which made me realise something interesting. The aircraft was a tricycle type and after the main wheels hit the nose splashed before the aircraft was tipped tail up. If you watch film of wheels down accidents from taildraggers, when the wheels dig in the nose goes down so the tail is almost over the nose before the nose builds resistance in the water so the forwards momentum easily drives the tail over. Tricycle gear aircraft are better for ditching.