1. (Shipbuilding) Any one of certain lines of a vessel, model, or plan, parallel with the surface of the water at various heights from the keel. [ 1913 Webster ]

☞ In a half-breadth plan, the water lines are outward curves showing the horizontal form of the ship at their several heights; in a sheer plan, they are projected as straight horizontal lines. [ 1913 Webster ]

2. (Naut.) Any one of several lines marked upon the outside of a vessel, corresponding with the surface of the water when she is afloat on an even keel. The lowest line indicates the vessel's proper submergence when not loaded, and is called the light water line; the highest, called the load water line, indicates her proper submergence when loaded. [ 1913 Webster ]

Water-line model (Shipbuilding), a model of a vessel formed of boards which are shaped according to the water lines as shown in the plans and laid upon each other to form a solid model. [ 1913 Webster ]