Boats float because of the principle of buoyancy, which states that an object will float if it displaces a volume of water whose weight is equal to or greater than the weight of the object. When a boat enters the water, the water it displaces pushes up on the boat with an equal force, preventing it from sinking. The hull's shape and size are designed to displace enough water to keep the boat afloat, allowing it to ride on the surface.