The Great Hunt

The Great Hunt is a quest in the Far Harbor add-on for Fallout 4.

Detailed walkthrough

 * The Mariner will tell about a mysterious creature that has been destroying boats leaving and coming into Far Harbor.
 * She will then ask to go to the boat and she will go to hers. Before boaring on, the player character is stopped by Avery, who tells it is an old myth and the player character is wasting time and putting themselves in danger.
 * The Sole Survivor will then board on the boat which has already been programmed by The Mariner to go to Red Death Island.
 * Once arriving, the player character has to look for the Red Death.
 * Talk with The Mariner about finding Red Death, and then it will appear in front of them.
 * Vanquish the Red Death.
 * The Mariner will be very disappointed, but will still reward with the Legend of the Harbor, a unique version of the Fisherman's outfit. She will ask if the Sole Survivor wants to tell the truth about the monster or lie. One will then get back onto the boat and sail back to Far Harbor.
 * The player character will then receive a hero's welcome from the town's people depending on if choosing to lie or not.

Bugs

 * When travelling to Red Death Island it's possible that one will get a black screen but still be able to move, but the Sole Survivor will land in water. To fix this one must activate the Vault-Tec Assisted Targeting System (travel to the destination and target the Red Death in V.A.T.S., or hold down the V.A.T.S. activation button.)
 * Possibly related to the above glitch, the player's boat may disappear, but the prompt to activate the boat's controls will still appear just off the northeastern coast of Red Death Island. The boat will be entirely invisible. Objects can still collide with the boat and the player is forbidden from fast traveling after activating the controls to return to the docks of Far Harbor. Do your best pixelhunting skills to activate the quest marker which will trigger the returning sequence. After a few seconds the screen will fade to black and you'll find yourself on a boat returning to Far Harbor.
 * Completing the main DLC quest line, may glitch the quest to make it unreceivable.
 * If you kill the NPC Machete Mike during the Hull Breach quest unprovoked, then you will be locked out of this quest (Though this could be due to The Mariner suspecting you were responsible for his death.