When sound pulses of ultrasonic frequency are transmitted into the sea, many echoes are returned to the source from scatterers distributed throughout the sea. Under the usual conditions of observation with echo-sounding and echo-ranging apparatus, the echoes merge indistinguishably. The resulting irregularly decaying sound is called reverberation (Eyring, et al., 1948) by analogy with the reverberation observed in rooms.
Raitt, Russell W.. 1948. "Sound scatterers in the sea." Journal of Marine Research 7, (3). https://elischolar.library.yale.edu/journal_of_marine_research/678