I was asking Adrian, but if you're working on this as well, great! I'll look forward to your build video and this new project.
By the way, I finally got a chance to test out my new hydrophones yesterday and no hum. After Mother's day lunch we went to the St Clair river below the Blue Water bridge and I managed to catch the freighter "American Spirit" as she headed up-bound into Lake Huron. I recorded the surface sound of the waves lashing the shoreline on two tracks, and the underwater sounds on another two tracks.
The hum is definitely gone now with this new set, but there are periodic pops and thumps which look like some kind of spike on the waveform. I manually removed them in Soundforge. Maybe just a property of piezo pickups.
I'll link to the audio when I get a chance.
Mark Day