WordReference Random House Unabridged Dictionary of American English © 2024son′ic depth′ find′er, - Nautical, Naval Termsa sonar instrument that uses echolocation to measure depths under water. Cf. Fathometer.
Collins Concise English Dictionary © HarperCollins Publishers::
sonic depth finder n - an instrument for detecting the depth of water or of a submerged object by means of sound waves; Fathometer