it doesnt float because it's not sealed.
just like carbon is lighter than oxygen, but it too doesnt float.
example: wood floats. a ship made of wood floats. but if you put a big hole in the hull, the ship sinks when water floods in, and only the loose bits will float free. if the ship were made entirely of wood and nothing else, it would sink until it reaches equilibrium, with the buoyancy of the submerged bits pushing up equaling the weight of the non-submerged bits pushing down. once you add nails battens, sails, hinges, hardware of every description, and all the other Non-Buoyant materials (mostly Treasure of course), it sinks.
i suppose if you were to put it in a balloon, displace the air with helium, and tie it off, the whole thing would float handily. i want to experiment with it now