I always find it interesting how the safety garus can sometimes look into the wrong end of the telescope. I recall the droll Boy Scout rule for packing eggs - pack eggs at the bottom so when they break, they don't get eveything wet!
Nothing can be nothing more depressing than making aircraft safer by making them float better when a critical failure occurs. It is even more depressing when the normal flotation system is expected to fail and let the aircraft capsize! So we enhance safety by putting a second flotation system on the aircraft!
OK enough sarcasm. How about we just require the normal floats to keep the aircraft upright? How about we require the design to have enough redundancy to assure that ditching was a very remote probability?