two ideas come to mind from the 'olden days' of electronics, one is the acoustic modems that used to warble when loading the programmes in an old sinclair spectrum type PC, the other is that before infra-red and radio based TV remotes there were a number that used ulttrasonic accoustic signals, both would provide a number of digital on/off channels.
Trouble is I doubt if the chips that they used are still made and getting microphones/speakers to work underwater would be a challenge, though there were systems that used the coil from a 'speaker agaisnt a thin steel sheet that then vibrated. That would also work as a microphone, though not sure how sensitive it would be. Hi-Fi quality is not needed however, so a unit could be made waterproof, as long as the depth wasn't too deep or there was a pressure equalisation method say with another rubber diaphram that tookup the change in volume due to pressure.
Ultrasonic transducers would be more tricky, though you could pinch some ideas from the pingers talked about here.
The ideal might be to get hold of the transducers from a boat echo sounder which would work at ultrasonics, but might be too directional.