Some years ago a New Zealand Warbirds society purchased a Catalina from the USA, and flew it back across the Pacific. Mid-ocean it had a multiple-engine failure and had to ditch.
Despite the fact that it was a flying boat, the ditching ruptured the hull, and it sank. Fortunatly eveyone managed to get into life rafts and were rescued.
So, if an a/c that is designed to float doesn't, I don't hold out much hope for those that aren't designed to float.