A damaged boat might damage the flotation device?

Not a bad idea,maybe you could use something like an airbag that deployed when water covered a sensor but you would have to be careful that it did not hinder any evacuation of the passengers. Maybe the airbag could be deployed inside the hull to block a hole.

The air bag would need to displace as much water as the boat w/airbag would normaly displace. Thats a big bag. The U.S. Marine Corps uses an emergency float on the CH-46 seaknights and it it takes up about 1/3's of each side ov the bird.

Some small boats have floatation built in- often as foam injected into empty spaces in the hull. This could be extended throught the boat, but works less well for larger boats.

I think the idea is very sound and believe the 'how it looks' factor my be creating creative barriers. If it could potentially save a life, then it has great potential. Anything that would also lower marine insurrance costs and deep salvage tasks is worth doing the math on. Though flotation foam is suitable on certain size boats theres not much that will prevent 60,000tonnes of steel going striaght to the bottom, I am interested to see where this concept ends up.