That depends on the volume of the brick. Whatever its volume is, its weight underwater is(weight of the brick in air) minus (weight of an equal volume of water)
another brick
A brick weighs about 3.5 to 9lbs.
it equals a half a lb.
The ships are not made of solid iron. They contain much empty space where the cargo and or passengers go. So the overall density of a ship: its TOTAL mass divided by its TOTAL volume is less than that of water.