Recording audio underwater
The easiest setups are those made with a pocket recorder connected to a cable and an hydrophone. The cable can be a few meters up to hundreds meters (longer cable legs usually need some extra signal conditioning, like a preamplifier, to drive the cable). Just like any other signal traveling along a cable, audio signals coming from hydrophones can be "packed" in order to have a higher immunity towards electromagnetic noise (see balanced audio). While EM noise immunity is usually not a problem underwater, it easily becomes an issue when part of the cable runs onboard of a ship (where radar, pingers, engines, inverters are present).
More complicate systems include audio interfaces for computers (from standard audio boards up to calibrated high frequency signal interfaces), interfaces for android and apple tablets, autonomous audio recorders (often called "bottom recorders" because they sit at the sea bottom and record the audio environment with monitoring purposes).