Archimedes' Principle states that:

* An object immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced by the object.

This means that when you submerge an object in water (or any fluid), the water pushes back up on the object with a force equal to the weight of the water that the object displaces. This is why some objects float – the buoyant force is greater than the object's weight.