If you're under attack from a serpent or otherwise have the chance, get to the mast of the ship before the boat breaks and you'll collect the nails and hide if you have inventory space. Of course you'll be left floating in the middle of the ocean yourself soon to die. but your gravestone floats.

Any cargo in your boat, whether ores or what-have-you will be left floating in a crate where the ship went down, so mark it on the map to return to pick it up if you don't have a death marker there.