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The numbers on a nautical chart represent soundings or depth data

Soundings are water depth measurements that indicate how deep the water is in a particular area in either feet or fathoms. A fathom is a nautical unit of measurement and is equal to six feet.

On a chart, sounding data with the same values are usually 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.

A nautical chart is a graphic portrayal of the marine environment showing the nature and form of the coast, the general configuration of the sea bottom, including water depths, locations of dangers to navigation, locations and characteristics of human-made aids to navigation, and other features useful to the mariner.

 

For more information:
Office of Coast Survey
A History of Charting America's Waters, NOAA 200th Anniversary Web Site
Diving Deeper Podcast, Episode 5 (Mar. 23, 2009) - What is a nautical chart?
Nautical Charts: A Message in a Bottle, NOS Education