I had experimented with contact mics and water before, sort of quasi-hydrophone. It worked ok as long there was some sort of current or splashing, running water, etc. The results were not great, even after upgrading my personal contact mics to a balanced design using dual piezos.
Enter the amazing “Let’s build some world-class hydrophones” Instructable by Jules Ryckebusch. His Instructable is extremely well documented, all the source materials are linked to and there are links to a couple of different ways to get the OPA Alice boards. I opted to order a few of the completed boards from PCBway to cut down on cost as I plan to build a few of these.
I made a couple of adjustments to the design by using a 1.5″ brass cap as my water ground, I used Cannare cable (cause the cotton strands inside add some extra strength for long underwater runs), and then I used some plastic shot glasses with the cup end cut off for the molds. I had one piezo showing thru after the polyurethane dried so I made a small batch of the A+B mix and repaired it. I read someone else who built these had the same issue; so if you plan to build some be aware before you go dunking yours.
Here are a couple of build photos of my hydrophones, but you should really visit the Instructable by Jules for the straight dope!
…and what would a hydrophone post be without a sound file. I recorded this one from the mouth of the San Gabriel River where Seal Beach meets Long Beach, Ca, using my Tascam DR40x.
The loud popping is shellfish and the clicking is dolphins. I am really amazed at the results.