This mission begins from where the last one ended. When you leave the cave, you will receive a call from Willis, who tells you he knows where your friend Keith is. Buck is usually in the bar at Badtown, so fast travel there and go into the bar. In order for you to get Keith back, you need to hunt down a special knife for him.
Buck wants you to find a boat that Vaas got to and get onto it. This boat is named the Nostalgia and it's marked on your map. It's in the middle of shark-infested waters, as well as surrounded by pirate boats, so you will need boat yourself. Go to the coast and search for one, then start heading for your target.
As you approach the Nostalgia, you will most likely be spotted by the enemies on board, so quickly make your way to the south side of the boat and use the ladder to get on board. You'll likely be swarmed when you get onto the ship, so your best option is get a good gun and be ready to start firing. There are two Snipers on board, as well as a Heavy Gunner, then the rest are just normal enemies.
Once all of the enemies are killed, go on the east side of the ship to find the door leading inside. Be ready for more action, as there will be an enemy right inside walking away from you, so do a Takedown on him and sneak towards the next room. If you have your Recurve Bow, shoot the one walking around first, then quickly shoot the other. If you kill them both quickly without making any noise, there's an alarm in the room that you can disable (it will disable the other, too).
Go down the stairs and open the door to go into the next room. There's one guy walking around, as well as one guy doing some welding around the corner, so use a Takedown on the guy walking around, then do the same with the other. Go through the next door to find three more enemies, two in the center and one of the far side.
If you have Chained Takedown, you should be able to kill all three very quickly. Otherwise, it's hard to Takedown all of them separately, so just quickly shoot them all. In the next room is your final objective, but when you open the door, you find a Heavy Gunner guarding it. Shoot him in the head until he goes down, then go ahead and hack the computer.
As you hack the computer, you find it is rigged up to explosives and it goes off, sending the ship to the bottom of the ocean. You need to escape quickly, so swim and hold in the sprint button while swimming to go faster. Follow the path until you get in a quick time event with an enemy. Once that's over with, keep going and you'll see an oxygen tank, so use that to refresh your air supply. When you reach the second one, use it and then go to the window to escape the ship and end the mission.
