Hydrophones are underwater microphones. There are a lot of uses for them from general interest for recording dolphins and whales to military applications such as sea mine detection.
There are a wide variety of hydrophones, ranging from cheap, simple ones for amateur usage, through to highly accurate, professional systems. Whichever you chose, it is always important to keep it in good working order and correctly calibrated. Using the TS602 Impedance Analyser/Hydrophone Tester you can test your hydrophone and keep it working effectively for many years of service.
Here is a short list of hydrophone sellers: