Fathometer

Origin

fathom("nautical length measurement used for depth") + -meter

Full definition of fathometer

Noun

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, 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.