I received 4x hydrophones for an upcoming experiment. I plan on aligning them with element spacing of around 15cm, bring the frequency down to 1-5kHz. It wont have a transmitter making it listening device only. 

Hydrophones are going to be used with experiments of detection where underwater sounds are coming from. 

Beam pattern would look like

Frequencies below 5kHz and above 2kHz would give a reasonable direction ability. Whale songs is in this range.

So when weather gets better, I will take my EVO8 (channel sampler), my laptop, on a field trip to the local harbour.