Basically a hydrophone is a kind of piezoelectric geophone which is designed to be used in water for recording underwater sound signal. Most hydrophones are based on a piezoelectric transducer that generates an electric potential when subjected to a pressure change, such as a sound wave. A standard hydrophone should be immersed in water, if it is buried in ground or put in a waterproof container, the hydrophone will give poor performance due to the bad acoustic impedance match.
Several hydrophones could be connected in serial so as to increase the signals from the desired direction but subtracting signals from other directions, which is usually called a hydrophone cable array or a submarine cable streamer. Most commonly, hydrophones are arranged in a line array and for reflecting signal at longitudinal (P-) waves but may be in many different arrangements depending on what is being measured.