An object immersed in a liquid experiences an upward force, called upthrust or buoyancy. This has been generally recognised for a long time.
It was the Greek mathematician and philosopher Archimedes (287-212 BC) who first put it into a general principle. His statement, now known as Archimedes' principle, was that
"when an object is immersed in a liquid the upthrust is equal to the weight of liquid displaced by the object".
Thus, a rubber duck floats because its mass is equalled by the water it displaces before it physically goes under the surface. A solid lump of iron on the other hand will sink, because the water cannot displace the mass of the iron before the iron sinks.
Why do steel ships float?
It's obvious that a piece of steel will sink in water, yet ships made of steel float. There must be some other factor that comes into play to explain this. Applying Archimedes' principle to the problem, the upthrust of the water must be greater than the mass of the ship, so somehow a great mass of water has to be displaced.
This is done by shaping the hull of the ship in such a way that as the ship sinks into the water it displaces more and more liquid until a balance is reached between the mass of water displaced and mass of the ship. This general principle applies to any object made from a material that is more dense than the liquid it is in.
