water line

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The line on the hull of a ship to which the surface of the water rises.
• noun Any of several lines parallel to this line, marked on the hull of a ship, and indicating the depth to which the ship sinks under various loads.
• noun A line or stain, as one left on a seawall, indicating the height to which water has risen or may rise; a watermark.

from the GNU version of the Collaborative International Dictionary of English.

• (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.
• (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.
• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun shipbuilding the outline of a horizontal section of a vessel, as when floating in the water.
• noun 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. 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.
• noun nautical 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a line corresponding to the surface of the water when the vessel is afloat on an even keel; often painted on the hull of a ship
• noun a line marking the level reached by a body of water