The weight of an object pulls it down into water. It displaces or pushes water aside. But if the object’s density (its weight in relation to its size) is less than the density of the water it displaces, it will float. That principle explains why a heavy wooden raft can float in water, while a small stone will sink to the bottom: the raft spreads its weight over a large area, while the stone’s weight is concentrated.
Boats, which are hollow, float because of this principle. The air inside them makes them less dense than they appear. Large ships that transport heavy material, though, have less air inside when they are carrying a big load. Such ships must be careful about weight limits and have load lines on their hulls that show how low they can ride in the water and still maneuver safely. Weight limits vary with the kind of water the boats are traveling through: they can carry more weight when in saltwater seas, which are denser than freshwater, and in cold water, which is denser than warm water.