Blood Tide

Blood Tide is a quest in the Fallout 4 add-on Far Harbor.

Detailed walkthrough
Cassie Dalton in Far Harbor on the docks wants you to avenge her family. There are three stages to this quest line. Cassie will pay you for your efforts and will be very grateful to you.

Cassie tells you about the illustrious history of the Dalton family on the Island and how the family has fallen to ruin. She is now the last living member of the Dalton family, which according to her was the wealthiest family on the island for generations.

First, she asks you to go to National Park campground and eliminate the feral ghouls there to avenge her cousin, Petey Dalton. The National Park is northwest of Far Harbor. Once the ghouls have all been killed you'll need to return to Cassie for the next step in the quest.

Second, Cassie asks you to avenge her grandfather, George Dalton, by travelling to Dalton farm at the northern edge of the island and killing the fog crawler there. The fog crawler can be a tough fight, so be prepared. The Dalton farm can later become a settlement. After slaying the fog crawler, fast travel back to Cassie at Far Harbor to receive her third step in the quest.

Third, Cassie requests that you avenge her husband, Danny Dalton, who was killed by the leader of a gang of trappers located at Brooke's Head Lighthouse, which is fairly far to the southwest, on the shore of the island. There are about a dozen trappers around the lighthouse, so you'll have to either fight your way through or sneak in the back from the ocean. Either way, once you've made it to the lighthouse, make your way up to the top of the winding staircase inside and at the top you'll find Douglas, the trapper who killed Danny. Kill Douglas and loot his corpse.

Return to Cassie at Far Harbor to complete this quest for some XP, some more caps, and a unique weapon called Bloodletter.