Definitions of water level:

• noun:   a water gauge that shows the level by showing the surface of the water in a trough or U-shaped tube
• noun:   the level of the surface of a body of water
• noun:   underground surface below which the ground is wholly saturated with water
• noun:   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