Waterline

The waterline is the line where the hull of a ship meets the surface of the water.

A waterline can also refer to any line on a ship's hull that is parallel to the water's surface when the ship is afloat in a level trimmed position.

[1] The load line (also known as Plimsoll line) is the waterline which indicates the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy.

The location of this base line varies on different types of aircraft.

Two typical alignments for the base line are the tip of the nose (negative WL are possible) or the "nominal ground plane" (measurements will be nonnegative).

Waterline of a ship. The mark above the waterline indicates the Plimsoll line
Water lines (WL) on a scheme of a fuselage. Baseline is at the ground line, all WL values are nonnegative