Water Depth

The vertical distance from the water surface to the seabed or another underwater feature at a stated time and position.

Actual water depth changes with tide, surge, waves and water-level variation. A charted depth is instead referenced to chart datum and must be combined with the height of tide or water level to estimate the depth available at a particular time.

A lead line or echo sounder can measure depth, but transducer offset, sound-speed assumptions, vessel motion and bottom detection affect the reading. Vessel draught is a separate measurement from waterline to the vessel’s lowest point and should not be treated as a second meaning of water depth.