The mission is unlocked once you purchase the Sonar Collections Dock in the northwest part of the map for $250,000. Once you do, and return as Michael, a woman will appear and asks you to find submarine parts that her husband drowned in (after you've been here as Michael, you can do this with any of the other characters). Get in the dinghy and look for the green section in the world map and head over there. Sonar will start activating so wait until the ripples set smaller and then dive down and look for the piece. There are 30 pieces to collect, but you don't have to do them all at once.?