I saw a post here recently on sending one through a port in the main camera housing, but it seems like a more promising approach would be to house one of the Zoom/Tascam/Olympus recorders in its own housing with a standard port to send a hydrophone through. Then you'd use a slate (or click two boltsnaps together) to synchronize the video with the audio and get rolling. Adds a layer of complexity for sure, but I can think of at least three advantages:
- Compared to a hydrophone on housed DSLR/CSCs, it means you totally eliminate focus motor noises
- You can position where you want for optimal sound. That means you could place several on several divers for action shots (caves come to mind), as well as being able to place it close to or away from regulators in order to capture/avoid breathing sounds, as per your wont. You could even put one up close to capture the regulator sounds and be able to difference them out.
- Assuming you can place them far enough away to matter given water's faster sound transmission, you might manage decent stereo, although the accoustics of doing that well are far beyond me.