A waterline is the line where the surface of the water meets the hull of a boat or ship. When a boat floats, part of it sits above the water and part sits below. The waterline marks that boundary. Naval architects paint a visible line on the hull at this spot to help them see how deeply the ship sits in the water.

The waterline changes depending on how much weight a vessel carries. An empty cargo ship rides high in the water, showing lots of hull above the waterline. When that same ship loads containers, it sinks deeper, and the waterline rises up the hull. Ships have maximum safe waterlines marked on them: loading cargo beyond that point can make the vessel dangerously unstable.

In shipbuilding, engineers design the shape of the hull at the waterline with special care, because this is where the vessel pushes through the water. A well-designed waterline shape means the ship moves efficiently, cutting through waves rather than fighting them. The famous clipper ships of the 1800s had carefully crafted waterlines that helped them race across oceans at remarkable speeds.

When something is described as below the waterline, it means hidden from view, like the bulk of an iceberg. An attack below the waterline on a ship is especially dangerous because water can flood in through any hole punched beneath the surface.