Question

an iron nail sinks in water, while large ships made of iron floats

Answer 1

Iron ship floats because it is constructed on the Archimedes Principla of Buoyancy.
It states that a body will float if it can displace t water up to its own weight.

Answer 2

Iron nail sinks because it has a density more than water whereas ship made up of iron floats on water because it has a density less than that of water as the volume is increased so the density decreases

hope it helps you