When it comes to standard audio recording, we rarely think of the medium that sound travels through, air. However, when we want to record something underwater, the material that the sound needs to travel through is suddenly much more important than if you were just doing a usual recording. Enter the hydrophone, which is basically a contact mic which has been prepared for use underwater.
The hydrophone comes with a basic set of requirements, the first of which is needing to be waterproof (obviously). Things such as frequency are also an issue when recording underwater, as many of the frequencies present underwater are of the higher variety. The speed of sound transmission is also much higher than through air, and as such sounds can separate into different frequencies similar to when they pass through metal. Recording a stereo sound is also difficult, as due to the speed and how sound travels underwater. As such you have to “fake” the stereo audio with small delays and level changes. Cable handling noise also becomes much more prevalent underwater as well.