water level

1. The level of the surface of a body of water.
2. Geology. See water table (sense 2).
3. The water line of a ship.

A simple device for establishing two points at the same elevation; consists of a water-filled flexible hose (from which air has been excluded) with a piece of glass tubing at each end; the water level is observed through the glass tubing.

The noun has 4 meanings:

Meaning #1: the level of the surface of a body of water

Meaning #2: underground surface below which the ground is wholly saturated with water
  Synonyms: water table, groundwater level

Meaning #3: 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
  Synonyms: waterline, water line

Meaning #4: a water gauge that shows the level by showing the surface of the water in a trough or U-shaped tube