|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 the Offshore 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.|
You begin the mission as Trevor, so get into a vehicle with Franklin and head to the marker. Stop to drop Franklin off (who the game switches to), and get into position on the balcony down the stairs. After a short scene, you'll be in control of Franklin and will have to kill two guards on the right of the ship.
Before you kill them, however, it's possible to clear the rest of the ship, without any time constraints. You want to shoot them in a way as to not alert any others, as well as not miss any shots. If you do miss, make sure you quickly hit the guard you missed before they alert the others. There should be around 10 guards, besides the two you need to kill, which you can kill in one shot with the right shot.
The two guards' deaths will make Michael board the ship and you will have control of him. Plant the bomb straight ahead, then go to each of the other points to plant more. If you did the above paragraph, there shouldn't be any guards in your way. The fourth, and final, bomb spot is inside of the cabin, which Michael will go in alone, putting you back in control of Franklin.
The alarm has now been set off, making guards aware of both Franklin's presence, as well as Michael's. First things first, pick off the guards on the ship, giving Michael some breathing room when he exits. Since they've spotted you, there will be plenty of enemies arriving in cars. Eventually, an attack chopper will arrive, so when it stabilizes, hit the pilot with a shot to take it out of commission.
Clear the rest of the guys on the ground, then Michael will begin to head back to the deck. There will be some enemies in his way, so you will need to kill them to give him a clear path. When he does, he'll jump off the rest and give you clearance to detonate the bombs using your phone.
With the ship's cargo now in the water, you'll regain control of Michael and have to find the device using sonar. Just follow the pings and it should be marked for you with an icon, and it also has a blinking red light on it. Once you locate it, the game switches control to Trevor, who's in the Submersible. Go to the device, go over it to pick it up, then bring it to the docks to have Floyd get it to land, which will be the end of the mission.