I have been trying out a hydrophone which I want to use as part of a sequence for a long-term project. It is a waterproofed piezo transducer with 3m cable terminating in a standard jack plug. First tests were OK but there were issues with noise and hum. Following a suggestion from a member of the Field Recording group on Facebook I used a Hosa MIT-129 transformer which has a standard jack socket input and a 3 pin XLR output. The difference in the recordings is amazing and the noise issues have gone. Moral of the story, always match the mic impedance to pre-amp input impedance!
Both tests were filling a wash basin with water, swishing it around and bit and then pulling the plug. Neither file has been processed.