One of the most important functions of a nautical chart is to show the depth and bottom characteristics through numbers, color codes and underwater contour lines. The numbers indicate soundings and show the depth in that area at low tide.
Soundings in white indicate deep water, which is why channels and open water are typically white. Shoal water, or shallow water, is indicated by blue on the chart and should be approached with caution using a depth finder.
Fathom curves are the wavy lines, and they provide a profile of the bottom.