After purchasing Sonar Collections Dock, go to the ? on your map with Michael and you'll meet with Abigail. She wants you to search for the pieces of the submarine that her dead husband went down with. This makes a Nagasaki Dinghy available at the docks, which has sonar equipped to it, and opens up the search for the Submarine Pieces.
Once you find all 30 pieces, go back to land and you'll get a call from Abigail, saying to meet back at Sonar Collections Docks.