Where the hull meets the surface is a critical boundary. The waterline is the line where the surface of the water meets the hull of a floating ship, and by extension the contour of the hull at that level.

Designers draw multiple waterlines in the lines plan to define hull shape. The actual waterline on a ship changes with loading, trim, and density of the water. Paint schemes often mark a boot-topping band around the expected waterline region. Fouling and corrosion patterns also relate to the waterline zone.

Observers use the waterline to judge draft and trim by eye. Submergence of certain marks indicates overloading. In heavy seas the waterline is constantly in motion as the ship pitches and rolls.

• Line where the water surface meets the hull
• Changes with load, trim, and water density
• Used in design and in visual draft assessment
• Related to load line and freeboard marks