waterline

A line formed by the surface of the water on the hull of a ship when she is afloat; any of a series of short lines marked on the hull to show where the waterline would be under different loadings

A horizontal line indicating the shape of an airfoil.

A line showing where the water has been, usually a line separating dry land and wet areas; a watermark or tidemark.

Down here along the waterline, the Enterprise has a belt of thick antitorpedo armor. It's not too promising.

The inner rim of the eyelid, just behind the lash line; primarily used in reference to the application of eyeliner.