Because of these specific gravities, the heavy person would actually float much easier. Considering these facts, one may still not quite understand how an object as large as a steel ship can float. In reality, the boat is not made out of only steel but actually an empty space encased in a thin layer of steel. Since the volume inside the steel is large and the total weight is small, the huge ship can float. The ship still displaces the same amount of water as the same size object completely filled with steel, but the empty space inside causes the hull to weigh much less than pure metal.
All objects have a center of buoyancy.
This is not necessarily the center of mass but rather the point at which the object's buoyancy is centered. In humans, the center of buoyancy is usually in the chest. A person will float when his or her center of mass is directly below the center of buoyancy. The closer the center of mass is to the head, the easier a person will float. The center of mass can be moved by moving one's appendages.