Question

why does ship floats but iron nail sinks

Answer 1

because ship is made design of fish and nail not

Answer 2

As the density of an iron nail is more than that of water, the nail sinks in water. The volume of water displaced is much greater than the volume of the iron ship. Hence, according to Archimedes' principle, the upthrust is more than the weight of the ship. That is why the iron ship floats in water.