11.7 Archimedes’ Principle

Archimedes' principle states that the upward buoyant force, that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid, that the body displaces and acts in the upward direction at the center of mass of the displaced fluid. 

The average density of an object is what ultimately determines whether it floats. If its average density is less than that of the surrounding fluid, it will float.