water line or wa·ter·line 
(wôtər-līn′, wŏtər-)
(wôtər-līn′, wŏtər-)n.
1.
a. The line on the hull of a ship to which the surface of the water rises.
b. 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.
2. A line or stain, as one left on a seawall, indicating the height to which water has risen or may rise; a watermark.
3. waterline The edge of the eyelid between the eyelashes and the eyeball: apply eyeliner to your waterline.