I always teach my students to think in terms of density when considering why anything rises within something else, like an inflated balloon within a sunken ship rising to the surface of the water. The reason is the object is less dense than the substance it is rising to the top of. In this example, a sunken ship is raised to the surface by means of inflating a balloon inside it, which forces out the water and replaces it with air, which is less dense than water. The ship rises to the surface of the water and, depending on the size of the balloon, rises above the waters surface. The only reason anything floats on top of anything else is because of density. The buoyant force has something to do with it, of course, but the real reason the ship comes to the surface is because of the decrease in density within its shape.