[n] a water gauge that shows the level by showing the surface of the water in a trough or U-shaped tube
[n] the level of the surface of a body of water
[n] 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
[n] underground surface below which the ground is wholly saturated with water; "spring rains had raised the water table"