Isn't it about time someone worked out a better idea for GA a la Full Aircraft Parachute?
How about inflatable bags in the wing spaces and/or the rear fuselage? Maybe permanently inflated bags or Ping pong balls? (Yes, I do know the reference.) It is not just the weight of the dinghy, there is the difficulty of getting it out and the difficulty of getting into it. How about an aircraft that stayed afloat?
Jemima is a Searey and our group experience is that, in accident where they fill with water or the hull is holed, they stay afloat for a while, varying on factors in the accident (usually an alighting accident.) However a couple have sunk after just a few minutes and I was concerned for my survival in cold water so . . . . .
I filled her up with polystyrene building board. Cost a couple of hundred bucks but I layered 2" board all down the aft floor glued with building adhesive and silicone caulking, and then braced it off the internal structure. Then I filled the nose with it, leaving a hatch and compartment for the battery. Extra weight came to about 15 lbs and displacement in water was about the 1100 lbs empty weight. I reckoned between the gas tank, air pockets and general buoyancy of some parts (Wing floats, tires and foam filled wing tips etc,) she'd float well enough to support at least my weight out of the water. I could have got a whole load more in but that would be a trade off against capacity, range etc. I did it based basically on guestimation but I am sure one could do a better and more exact calculation and design quite easily.
Of course you certified types are screwed but us 'Experiemental" or "Amateur Built" can do it easily. There was no paperwork involved in stuffing Jemima with Polystyrene!