Depth finder and Fathometer

Depth finder noun - Navigational instrument used to measure the depth of a body of water (as by ultrasound or radar).

Fathometer and depth finder are semantically related in sonar topic. In some cases you can use "Fathometer" instead a noun phrase "Depth finder", when it comes to topics like echo sounder, sounder.

Fathometer noun - Depth finder for determining depth of water or a submerged object by means of ultrasound waves.

Depth finder and fathometer are semantically related in sonar topic. You can use "Depth finder" instead a noun "Fathometer", if it concerns topics such as echo sounder, sounder.