The Mariner will give you this quest after completing a series of optional quests for her, right after Hull Breach 3. Follow her advice and take whatever powerful weapon you have and meet her on the dock. Activate the mission so you’ll know the boat you should be boarding on. Step on the boat and you’ll have the option to travel to Red Death Island.
Talk to the Mariner and she’ll give you the Legend of the Harbor outfit. Vanquish the Red Death then sail back to Far Harbor. Head towards the Mariner and receive a hero’s welcome from the townspeople. Talk to her afterward to complete the quest.