Archimedes and the principle of flotation ... "Cast iron sinks" ... unless it's a ship, of course is an old joke.
What has to happen is that supposing the object has a mass of 1kg, then it has to displace 1kg of water to float. However the volume of the object is decided by the object's density. Unless, that is, it is "hollowed out" or otherwise manipulated, such as is done in the hull of a metal ship ...
There's a thing called a "Eureka can" used in physics labs.
