How do huge steel ships float?

Steel is about eight times denser than water, so a solid block of it sinks. A ship’s hull, however, is a hollow shape filled mostly with air, so its average density is lower than that of water. As a ship settles into the sea, it displaces more and more water until the weight of that water equals the ship’s own weight, and then it floats. Seawater is denser than fresh water, so ships float slightly higher at sea than in rivers. Load lines painted on hulls, known as Plimsoll lines after British politician Samuel Plimsoll, show how deeply a ship can safely be loaded.