point the hydrophone's arrow to 180° with your e-motor running.
Place your cursor on the upper end of the vol control (the game cursor will turn red when you're on the correct spot), clicking the left button on your mouse until you hear the vol increase.
Now, come to "All Stop". With your engines stopped click onto the hydrophone arrow and slowly drag it thru 360°...eventually you'll begin hearing the chugging of target ships.
Caveat: make sure you're below 10m when you do this or it won't work properly.
