Displacement is what makes boats float. Displacement was discovered by Archimedes, and the Archimedes Principle is also used to describe the theory of displacement.
The weight of the boat displaces an amount of water that equals its weight, which is how displacement works to make a boat float. The shape of the boat keeps water from filling it.
For an example of a boat displacing water, imagine a small boat that weighs 50 pounds. The boat sinks into the water until it displaces 50 pounds of water. At that point, 50 pounds of pressure are then pushing upward on the boat from the water, making it float.