water level

noun

1.
the level of the surface of a body of water
drawdown
2.
underground surface below which the ground is wholly saturated with water
groundwater level water table
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
water line waterline
4.
a water gauge that shows the level by showing the surface of the water in a trough or U-shaped tube