Water depths are measured by soundings usually acquired by hydrographic surveys. The depths may be in either feet or fathoms (the chart will indicate which). A fathom is a nautical unit of measurement and is equal to six feet.
On a chart, water depths may be connected with a line known as a depth contour, similar to the topographic lines or surface features that you see on a map. Depth contours present a picture of the bottom to the mariner.
In addition to showing water depths, a nautical chart also tells mariners about dangers to navigation, aids to navigation, anchorages, and other features. U.S. Chart No. 1 explains all the numbers and symbols found on both paper and electronic nautical charts.