Floating and sinking

Whether a thing floats is not about how heavy it is. A huge steel ship floats and a small pebble sinks. What matters is how heavy it is for its size: its density, compared with the water around it.

A thing sitting in water pushes some water out of the way, and the water pushes back up with a force equal to the weight of the water moved. If that push is as much as the thing's own weight before it is fully under, it floats. A ship's steel is shaped round a lot of air, so it moves a great deal of water.