The displacement of water is an excellent way to measure the volume of a solid provided that the solid does not dissolve in water and that it does not float.  If the object floats then it will not completely submerge itself in the water to fully displace it.  One method to measure the volume of an object that floats in water is to weigh it down with a heavy object.  The important thing is to first measure the water level with the heavy object submerged in it, then measure the water level with the heavy object holding the floating object down under the water.  Find the difference between the two levels and that is the volume.