The principle that explains why a boat floats in water is known as Archimedes' principle. This principle states that an object submerged in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced by the object. For a boat to float, its overall density (mass divided by volume) must be less than that of the water. This balance between gravitational forces and buoyancy allows boats, even those made of heavy materials, to remain afloat. Understanding this principle is key in fields such as hydrodynamics.