The water taxi that flipped over on the east coast didn't sink. Also, most cabin boats will float for quite some time after being swamped.

The only real benefit of an unsinkable hull is that if you tie yourself to the boat while it is swamping, the Coast Guard will find your cold, dead body right away, and your wife can get your insurance money without having to prove your death.

It's not necessarily drowning that kills in saltwater. It's hypothermia, or drowning because hypothermia has numbed your muscles. In the unlikely event that the seas are very calm and your boat fills with water, I suppose you could stand on a high point and stay dry.