Question

why a ship made of iron and steel does not sink in water , but while the same amount of iron and steel in the form of sheet would sink?

Answer 1

A ship made of iron and steel doesn't sink in water, but while the same amount of iron and steel in the form of sheet would sink because it is not completely solid and has full of air filled in it.

Answer 2

this phenomena behind the Archimedes ' principal .
when a body submerged wholly or partially in a liquid at rest , it experiences an upthrust . the upthrust is equal to the weight of the liquid displaced by the submerged part of the body .

according to this ,

if weight of body greater then the upward force or upthrust by the liquid acting upward , then body sinks
for iron or steel in the form of sheet same phenomena occur so, this body sink in water .

but in case of iron or steel in the form of ship , its weight is equal to or less then the upthrust or weight of body is just balanced by upthrust , then ship float or doesn't sink in water .