Why iron nail sinks but iron ship floats?

Water has weight. If you push something into water, all the water above it pushes down on the top and the top pushes up with an equal force. Same at the bottom, but there is more force at the bottom than the top, so there is more force up than down. This is called buoyancy. It only depends on the amount of water the thing displaces, not how much the thing weighs. If you beat a nail into a bowl so it displaces more water than its own weight, the bowl floats.