any one of certain lines of a vessel, model, or plan, parallel with the surface of the water at various heights from the keel.-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.