Why is everyone so surprised a ship made out of concrete would be able to float?
Last time I checked, steel isn't all that buoyant, either. Drop a crowbar into a pool and see what I mean.
The material used to make the hull has nothing to do with whether it floats or sinks. It's how much water is displaced by the volume of the ships hull. If the amount of water displaced is greater in weight than the weight of the ship, that ship will float.
Heck, back in WWII, the Royal Navy wanted to make supply ships and aircraft carriers out of icebergs!
