To start this quest, head to the docks between Harbormaster Hotel and Shamrock Taphouse in East Boston. Look for a kid standing at the edge of a dock. He's fascinated by what he perceives to be a sea monster in the distance. Using the quest marker, swim to the location of this alleged sea monster. It turns out that it's the Yangtze, a Chinese submarine.
Head inside the sub and look for Captain Zao, who needs help repairing his sub's reactor. Assuming you agree to his request, your next destination will be Saugus Ironworks at the northeast corner of The Commonwealth. Prepare for a long fight against the factory's current residents, the Forged, a pack of Raiders with a fondness for flame-based weapons.
Travel up and around Saugus Ironworks toward the quest marker until you reach the dampening rod sleeve. Grab one of the dampening coils inside the sleeve. Then leave the factory. You'll find an exit near the dampening coils, so you won't have to backtrack to the door from which you originally entered.
Return to the Yangtze and update Captain Zao. He'll now ask you to take on another fetch quest. Don't worry: This one is within the lower bulkheads of the sub. Go there and take down his former crew-turned-Ghouls. The quest marker will lead you to an intercontinental ballistic missile. Take out the warhead in the ICBM.
Head back in the direction of Captain Zao, and you'll find a spot where you can activate the reactor core. Install the dampening coil and then the warhead. Head back to Captain Zao to complete the quest.