fathometer

From fathom (“nautical length measurement used for depth”) + -meter.

fathometer (plural fathometers)

1. (nautical) A depth finder that uses sound waves to determine the depth of water.
   • 1977, Richard O'Kane, Clear the Bridge: The War Patrols of the U.S.S. Tang, Ballantine Books, published 2003, page 347:

     A Fathometer is just a vertical echo-ranger; its pings would sound the same and could drive a submarine out of position just as effectively as horizontal echo-ranging and might even make a contact.