sonic depth finder vs fathometer

sonic depth finder and fathometer both are nouns.

Word	Noun	Adjective	Verb	Adverb
sonic depth finder	Yes	No	No	No
fathometer	Yes	No	No	No

As nouns, sonic depth finder and fathometer are synonyms defined as:

sonic depth finder and fathometer: depth finder for determining depth of water or a submerged object by means of ultrasound waves

sonic depth finder (noun)	fathometer (noun)
depth finder for determining depth of water or a submerged object by means of ultrasound waves	depth finder for determining depth of water or a submerged object by means of ultrasound waves