Hydrophones are too often overlooked as tools for field recording or sound design, even more so than contact microphones. I guess the reason for his is that (1) they're so unpredictable and (2) no one wants to realistically portray underwater sound. Hollywood and by extension the media in general will happily throw a lowpass filter on noise and add some bubbling sounds and that's about the extent of underwater ambience that we'll get.

I've had a pair of JrF hydrophones for a couple of years and only went out to record with them a few times last year. Luckily I was reminded to listen back to my stuff from last year while discussing hydrophone recording with @lucafusi. He posted a couple of hydrophone recordings that sound really sparse and subtle. When I was recording the Union Canal in Edinburgh on a warm summer day I could hear loads of tiny clicks and noises, but never listened back to this after I copied the files to my library. What better excuse to start sifting through the archives?