If you dropped a small stone into the water, it would sink immediately. But if a block of steel weighing thousands of tonnes were made into a ship, it would float. Any object placed in water experiences two forces: gravity, which pulls it down because of its mass, and the buoyancy force, or upthrust, which pushes it up. When these forces are balanced, the object floats. The buoyancy force of a stone is much smaller than its gravity force, which is why the stone sinks (although the buoyancy force makes the stone sink slower than it would do through the air). A ship also experiences these two forces. But because of its body design, which contains a lot of air, it displaces (pushes aside) enough water so that the buoyancy force is equal to its gravity force. That is why a ship floats.

In fact, a ship does not float on the surface of the water; it sits in the water with part of its hull below the surface. The heavier the load, the deeper it sits. The deeper into the water the ship sits (without actually submerging completely), the more buoyancy force is created. So if a ship weighs 1000 tonnes, it will sink into the water until it has displaced 1000 tonnes of water. Provided it displaces 1000 tonnes before it is completely submerged, it still floats. But if the ship weighs more than the total volume of water it displaces, it will sink.