Question

Why iron nail sink in water and iron boat floats

Answer 1

The density of nail (as of iron) is much larger than the water. So it sinks easily. The weight of the water displaced by the ship is equal to its weight, so it floats. Whereas the weight of the water displaced by the iron nail is less than its weight so the iron nail sinks

Answer 2

Iron nail sinks because the volume of water displaced by it is less than its own volume.

Boat floats because the volume of water displaced by it is more than its own volume.