The problem with piezoelectric and contact mics is that they are not well matched to typical audio inputs. The reason why these devices often sound tinny is because the piezo sensor presents its signal through a series capacitance which is small, typically 15nF or less. When wired to a normal 50 kilohm line input this forms a 200Hz high-pass filter, which eliminates the bass. Because the piezo disc has a very high impedance (1 MΩ typical), it should be buffered to avoid possible impedance mismatch with an existing audio system. This circuit acts as a impedance buffer, and some amplification does not hurt as long as it is around 40dB. The best piezoelectric sensors for listening is based on Aluminum Nitride and molybdenum. PZT-5 also known as SensorTech BM500,Channel 5500,Morgan PZT5A1 is widely used.

The diodes are there to avoid overloading the circuit if it is dropped or bumped in to something hard.

NOAA technical memorandum NMFS 2008NOAA-TM-NMFS-SWFSC-417 also known as A guide to constructing hydrophones and hydrophone arrays for monitoringmarine mammal vocalizations, describes how to build a single crystal hydrophone.

If the piezo electric crystals are a suitable distance and 45° degrees apart from each other in a flat circular array with 8 piezoelectric sensors, they should be compatible with Manyears or ODAS.

Approximate values for fresh water and seawater, respectively, at surface pressure are 1450 around 1500 m/s. The speed of sound in water increases with increasing pressure, temperature and salinity.

Also, read: https://github.com/Supermagnum/passiveSonar

Github: https://github.com/Supermagnum/xSoundsMicrophones/tree/master/Hardware/Microphone-piezo