waterline

plural waterlines

1. nauticalA 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
2. aeronauticsA horizontal line indicating the shape of an airfoil.
3. A line showing where the water has been.

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

   1992, Neal Stephenson, Snow Crash, page 391

   Synonyms: watermark

4. The line formed by where the water level of a body of water currently is: where the water's surface currently meets the bank; it may move higher or lower depending on the weather, tide, or season.

   Near-synonym: water's edge

   1 more example

   We are walking south-west, down a gently increasing slope, with the old farm buildings of the tiny hamlet of Villemiers [in the department of Charente] visible on the other side of the valley a kilometre away. The Transon [a tributary of the river Charente] meanders in the bottom of the valley below, a lazy trickle of a stream with small muddy pools at intervals, home to numerous coypu, a South American rodent that escaped from fur-farms long ago and has found a home-from-home in the many rivers and lakes of the Charente. They are semi-aquatic, resembling beavers in all but their long, rat-like tails. They can be something of a nuisance, as they are great burrowers, creating huge holes in the banks just on the waterline, which does little harm in a stream, but can be disastrous in a man-made lake, since their burrows can puncture the dam.

   2015, Dave Goulson, A Buzz in the Meadow: The Natural History of a French Farm, Picador, →ISBN, page 14

   Synonyms: water's edge

5. cosmeticsThe inner rim of the eyelid, just behind the lash line; primarily used in reference to the application of eyeliner.
6. A water pipeline.