While exploring Far Harbor, the Sole Survivor may randomly pick up the Shipbreaker's radio frequency, immediately starting the quest. Provided settlements have been built in Far Harbor and settlers were recruited, any of them may approach the player character to tell them about Shipbreaker, starting the quest as well. The location of Shipbreaker, a unique fog crawler, will depend on where the player character picks up Shipbreaker's signal. Known locations for Shipbreaker include north of Briney's Bait and Tackle, halfway between Atom's Spring and the Nucleus, and just outside of the Oceanarium. Once the fight is over, the player character is prompted to speak to Old Longfellow. He will talk about how he has been searching for Shipbreaker for years. Tell him that Shipbreaker is dead, and the quest will complete with the player character earning Skipper's Last Stand, a unique harpoon gun that Old Longfellow was saving to deal the final blow on Shipbreaker with.
pc If a player doesn't track it down during its original appearance it may vanish with the message "The shipbreaker has moved on for now". It will then fail to reappear and its radio signal will be blacked out even though it will still lead you to the quest location. [verified]
This can be fixed using the console commands on PC. After travelling to where the Shipbreaker should be type in showquestvars DLC03WorkshopRadiantOwned05 to get the Shipbreaker's ID. Then prid whatever the id is. Once this is done it should be showing at the top of the console. Then type disable, press enter then enable and press enter again. Once the console is closed, the shipbreaker should spawn and you can kill it and finish the quest.
The game will sometimes bug and the radio signal will disappear permanently, if this happens you can use the following console commands to find and reveal Shipbreaker: player.moveto xx040c3a, prid xx040c3a, disable, enable where xx is you load order number (normally 03).
pcps4xboxone The location can still be tracked from the radio menu even though it is blacked out. Scrolling down to the location of the radio frequency, will still lead to a location regardless of the quest monster being there or not.
pcps4 The signal will never appear while on the island. Using the command sqs xx040a87 will reveal that the quest is Stopped. [verified]