With the Tide

Once inside, you just need to follow the straightforward path until you reach freight elevator. You'll find various enemies along the way but at this point, you should be more than equipped to deal with them.

After reaching the elevator, turn it on only to see it stop. Jump down to the elevator and through the shaft to reach the lab. Now, search the highlighted areas for clues.

After searching for clues, open the next door to find Harlow. For the first part of the mission, just deal damage to Harlow to prompt her to inject herself with a mutagen. Now you'll have to fight outside the lab. She only has two attacks; one with her chainsaw and she'll throw a molotov at you. Run around and keep shooting at her to drain her HP.

Once enough damage is dealt, you'll have to pick up the mutagen in the next room to enter group fury mode. Use this ability to deal damage and kill Harlow. Kill the remaining enemies in the area and make your way to the boat. Now you can fast-travel from here to do the remaining stuff you need to do. Entering the boat and leaving the island will end your quest.