Question

an iron ship floats on water but an iron sheet sinks in water why

Answer 1

because iron ship has streamlined structure

Answer 2

The iron sheet sinks in water because the density of iron sheet is more than water.

Now look, iron sheet is thoroughly made of iron and does not have any cavity and the water displaced by the iron sheet is less than the weight of iron sheet. Since the mass of iron sheet is more than the mass of water displaced then force of gravity i.e downward force is more than the buoyancy ie. upward force. So the iron nail sinks

while.....

The mass of water displaced by the ship is much more than the mass of ship so the buoyancy i.e upward force is more in this case than the gravitational force i.e downward force. So the iron ship floats on water.

And because of this simple iron A heavy iron ship floats on water but a light weight iron sheets sinks water.