Cruise ships float on the water because of a principle called buoyancy. According to this rule, an object will float if the weight of the water it pushes aside is greater than or equal to its own weight. When a cruise ship is in the water, it displaces a certain amount of water, which weighs more than the entire ship. The weight of the water that the ship displaces is called the "displacement" of the ship. This displacement is greater than the weight of the ship itself, so the ship floats on the surface of the water.