Can a MEMS piezo microphone perform as an alternative to a hydrophone?

There are currently over a billion miniature microphones produced every year to feed the global market for smartphones. There is a growing trend to develop smartphones with water-resistant capabilities. One of the significant points of weakness for a water-resistant smartphone is the microphone. A capacitive microphone can be very susceptible to both dust and water egress. This has motivated the development of cheap piezo MEMS (micro-electro-mechanical system) microphones, which are less susceptible to dust and water egress. As a selling point, the manufacturer of the Vesper piezo MEMS microphone indicates that the device can work underwater as a hydrophone. Small, cheap, low-power and reliable MEMS hydrophones could prove incredibly useful in the underwater domain, enabling a variety of IoT (internet-of-things) type applications. We test the Vesper piezo MEMS microphone in an underwater environment to assess the manufacturer's claims with regard to its use as a hydrophone. We compare both the sensitivity and the directionality of the microphone to a standard hydrophone.