Here There Be Monsters

Here There Be Monsters is a quest in Fallout 4.

Synopsis
At Boston Harbor, a small boy named Donny weaves tales of a sea monster lurking in the bay. The visible long neck of the monster gives credence to his claims, but not all's as it seems...

Walkthrough

 * The player has to talk to Donny to start the quest and then swim to the 'monster' and climb on top of the sail (the "head" rises periodically to help the player keep their bearings). Turns out it's actually a Chinese submarine: Yangtze-class, vessel number 31.
 * Power armor is useless for this, as it sinks to the bottom. Don a hazmat suit or pop some Rad-X before swimming to minimize the danger.
 * Navigating through the derelict interior, the player will eventually find its Captain, Zao. Two centuries of being stuck in the bay after launching the missiles and running into a sea mine have given him a decent grasp of English on top of the intelligence he already had. Unfortunately, Zao is the only sane ghoul on the ship. He will explain his predicament and request that the player aid them in restoring the Yangtze to full functionality.
 * The first part of the solution requires the player to retrieve the nuclear dampening coil found at Saugus Ironworks. This requires fighting through Forged into the blast furnace and collecting it from the apparatus up the catwalks. The second involves navigating the foredecks of the submarine, fighting through the ghoulified remnants of the crew led by the feral First Mate, and reaching the missile room where a single nuclear warhead remains unlaunched. The processed fissile material inside can be cannibalized to power up the reactor.
 * Finally, the player has to enter the reactor core (heavily irradiated and place) the coil to stabilize the core, then insert the warhead material inside. The player should not put the warhead material inside without the coil, at least not without saving, as the core will melt down and there goes the neighborhood!
 * Report to Zao for the reward. Zao will be jubilant and reward the player with a good amount of gear (but sadly, not his uniform) for their assistance, together with three homing beacons to use with the remaining payloads onboard the Yangtze.