You do know how loud the BlueROV2 is, Right? You won’t be able to hear anything while the thrusters are armed. Even the stepper motor in the Ping360 sonar can be heard by my GoPro camera.
I own a Aquarian Audio H2a-XLR, but their hydrophones are only rated (tested?) up to 100 KHz.
Maybe you could e-mail them and see if their “scientific” version would work above 100 KHz even if it’s not officially rated for it. looks like there’s still some response above 100 KHz on the graphs.
Audio from mine of a dock splashing in the waves:
