Location: You need to purchase a property called Dock Sonar Collections (it costs 250 thousand dollars) located in the North-Eastern part of the map, Westward of Mount Chilliad. After you purchase the property, while playing as Michael wait for a question mark to appear, reach the place and walk over to the other end of the pier shown in the above screenshot. Talk to the woman that you find there. Abigail will send you on an errand to find parts of the submarine deep underwater in various parts of the map. To start the process of searching for the submarine parts, walk aboard the red boat (Nagasaki Dinghy) moored at the pier, which is equipped with a diving suit.
To locate the submarine parts, you may either use the "GTA V - Maps and collectibles" guide, or head towards the green circles appearing on the world map, and follow the radar's readings. I recommend that you have a careful look while underwater, because apart from the collectibles themselves, you can also find briefcases with cash, first-aid kits and new weapon models. You should watch out for sharks also, and steer clear of them, because they may attack Michael.
After you have collected all thirty parts, wait for Michael to contact Abigail and return to the Sonar Collections dock to talk to the woman. The final to this conversation will be a pretty disenchanting one because you will receive the measly 10 dollars for your efforts. Depending on your preferences, you can either exact revenge on the woman, or let her leave the dock.
Rewards for the completion of the mission:
- New contact on Michael's contacts list: Abigail
- 10 dollars (you receive the reward after you have found all the elements of the submarine)