These guys tout an unsinkable sailboat. Boat inside a boat with foam in between. Apparently you can punch a big hole in it, flood the inner compartment and it will still be able to sail. I suppose if you snap off the keel then it could go upside down, which isn't very good to you even if it floated. So really "unsinkable" means more than the ability to float.
http://www.etapyachting.com/