The Harpoon Mission can be accepted after you complete the National Treasure, Knuckleball, and Room Service quests. In the mission, the room will slowly fill up with poison, and you need to find a way to turn it off so you can get to Admiral Benitez.
Turn off the 4 Valves
As you enter the room, the doors behind you will lock and soldiers will spawn and poison will start filling up the room. After killing the soldiers, you need to turn off the 4 valves in the room to turn off the poison.
- The first valve is just below the platform where Admiral Benitez is standing.
- Go up the stairs and go down the hallway. As you reach the end, jump off the platform to get to the second vault near the room where Admiral Benitez is.
- Head to the right and climb the ladder to turn off the third vault.
- Jump through the platforms and pipes to get to the last vault across the room.
Be sure to come prepared because the poison will deplete your health quickly while you are trying to turn off the valves. You have to quickly kill the soldiers in the room as well to avoid further damage.