George Spanos has published a very cool article talking about hydrophones and how to record underwater sounds with them. You can also listen to some the sounds recorded.
Ok, so now that the basics are covered where you may ask can one acquire one of these underwater recording devices? I used the Aquarian Audio Products H2A-XLR for the following recordings. The device itself is very light but heavy enough to not move around in water, which is important or you will pick up noise from the microphone cable.
The hydrophone terminates in a balanced XLR which can interface with any recording device that can supply +48V of phantom power. I used my handy Zoom H4 which I find is the most portable and quick to set-up piece of recording gear I own. Sure, the screen is small but if you monitor your recordings carefully through headphones that should not be a problem.
I was inspired to make these recordings from a blog post I read over at Noise Jockey. Nathan made some great recordings that you can hear via this link.
UPDATE 1: Richard Devine spreads the word about another hydrophone recording he did with DPA 8011 Omni Microphone.
Recorded on May 6th in Lake Lure North Carolina. All recordings taken from the lake, rivers, and dam of Lake Lure. Everything was recorded at 24-bit 96k with a Sound Devices 702 recorder and DPA-8011 Hydrophone. No editing or post processing.
UPDATE 2: Some days ago I saw a very cool post on Noise Jockey, where Nathan uses another Hydrophone, this time not for underwater sounds, but a different way by using a magnet with it. Check:
It is what is says, people! ;-) I ducked out some handling noise, but for the most part the audio is unaltered. Enjoy.