The density of the VOLUME of the ship is less than the density of the water it displaces (pushes out of the way). While the steel of a steel hulled ship is denser than water, the steel plus the air enclosed by the steel is less dense.
Surface area.
A stone has a greater density than water; a ship, less.
a ship
The basic reason that a ship floats is that the enclosed volume in the ship's hull is greater than the volume of water which has the same mass as the ship.
The ship floats because of its large volume which displaces an amount of water that is more than its weight. That creates an upward force called buoyant force which keeps the ship on the surface.
Weight of ship = weight of (displaced) water.
PLANKTON
a ship floats on the water and a submarine can go underwater and on top of the water
Because the weight of the water that they displace is greater than the weight of the ship.
a green liquid
The principle of Displacement gives 'rise' to buoyancy. When a ship is put upon water with a cargo weight less than the weight of water that the fully loaded ship displaces, it will float.
Tides.