Floating or Sinking? - Displacement of Water

Experience teaches us which objects float and which ones don’t. A ship floats, a metal weight sinks instantly. That’s obvious. Nevertheless, it is kind of interesting:

Most parts of a ship are made from metal, too - so why does it float?

An object in water is subjected to two opposite forces: On the one hand, gravity pulls it downwards. On the other hand, a lifting force is affecting the object as soon as it is submerged in water (see buoyancy). Whether the object floats or sinks merely depends on which force is stronger.
The force of gravity is nothing but the weight of the object.
According to the Archimedes’ principal, buoyancy is as strong as the weight of the liquid that was displaced by the volume of the object.
If the weight of the displaced fluid is heavier than the weight of the object, the buoyancy is stronger than gravity. If the displaced fluid is lighter than the object, gravity is stronger. Since the weight of a certain volume is heavier the bigger its density is, floating or sinking depends on the density of an object.

So what floats?

The density of wood or polystyrene is smaller than the density of water; these things float. The density of a rock and of most metals is bigger than the density of water; these things sink to the ground.
However, a third case exists: If the density of an object is the same as the density of water, it hovers in the water.

The special design of ships is the reason why they float: The hull contains much hollow space. First, this reduces the density of the ship and secondly, it displaces an especially large amount of water. So despite of its heavy weight, the ship becomes lighter than the water, which was displaced by it and the ship floats. However, as soon as water leaks into the hollow space, the ship starts to sink.

Fish are able to adjust the density of their bodies to the density of water, which enables them to hover at a certain depth. For that reason, they have an air bladder filled with gas. If the amount of gas is increased, the fish floats to the surface; if it is reduced, it sinks.
Submarines use a similar system for surfacing and submerging.