Usually, the hull of a ship contains a large volume of air. why?
lots of points: )

The shape of the ships hull causes the ship to displace (get rid of or remove from one place to another) a greater volume of water then a solid piece of steel (or metal whichever the ship is made of) with the same mass. A ship displaces a volume of water equal in weight to the submerged portion of the ship, ok.
in other words The air reduces the ships overall density so it can float!
hope this helps!