The SONAR implant is popular among workers in the Abyss because of the ability to see in the inky blackness of the endless ocean. It is a processing module that links into an existing aural implant to allow processing of ultrasound, and a vocal implant that can send up strong ultrasound waves, to allow for echolocation.
Echolocation is reliable up to about 100 meters, after which the soundwaves are too dissipated and scattered to be reliable.