The numbers on a marine chart represent soundings done by agencies such as the National Oceanic and Atmospheric Administration, or NOAA. The international unit of measure is the fathom, which is equal to 6 feet. Soundings are the measurement of depth at low tide, usually taken by the marine equivalent of GPS technology. The numbers on the chart represent the sounding results in fathoms, which gives the depth in feet when multiplied by six.
Matching the ocean depth to the draft of a vessel keeps that ship or boat from running aground. The draft is the measurement from the waterline to the lowest part of the boat, usually the keel. Maritime charts are also marked with known underwater hazards and restricted areas.
For example, looking at a chart for Hawaii, there are a number of smaller islands on the western end of the chain where the waters and those islands are restricted. The entire zone is outlined and marked "Area To Be Avoided." In this case, the area is part of the Hawaiian Island National Wildlife Refuge, which may only be visited with a permit. The depth information is still provided.
For the main island of Hawaii, the chart shows colored blue areas next to the shore. These are the shallows, with the depth numbers shown. Shallower depths are usually colored. Similarly, the same types of colored areas and numbers surround the other islands.