|As with The Jewel Store Job, there are two different approaches to this mission, with both resulting in completely different walkthroughs. Both will require you to finish the Minisub mission, while this approach requires one additional mission (Cargobob) to be finished. Once all of the setup missions are complete, an "H" will appear on the map at Floyd's place.|
After the debriefing at Floyd's place, get into a four-door vehicle and head to Sandy Shores airfield, which is a long drive. Once you get there, get in the Cargobob and you'll need to pick up the Submersible. Hover over it and then have the hook come into contact with it, which should make the connection.
Now, you need to fly the Submersible to the target location and drop it off, where you will switch to Trevor. Bring up your phone and go to the Trackify app, where you will need to hunt down the item you're looking for. Make it so the red blip appears in the north and then keep heading to that direction.
When you get to it, just run into it on the sea floor to pick it up, then make your way to the surface. Switch to Michael and then pick up the Submersible again, going back to Sandy Shores. However, on the way, you'll encounter some Merryweather guys gunning after you. Switch to Franklin and begin firing at them.
At first, it will be two boats and the second wave will include more boats and a helicopter. After that, there will be two more waves of two choppers each, so switch to the side with the helicopter and keep firing until they go down. Switch back to Michael and fly back to the airstrip, letting go of the Submersible close to the ground and then landing to finish the mission.