The Tannery has two areas, one interior and the exterior of the building.
Exterior
The second floor office by the conveyor belt has an Expert Safe and a Caps Stash on the table. The docked boat in the west side of the building has some lootable containers inside but has several ghouls, including the sturdier Charred ghouls. You may also find a lone trapper getting swarmed by the ghouls.
Interior
Inside the tannery, there are two ghouls in the first room behind an Advanced locked door. You can pick the lock yourself but alternatively, you can get the key from a bloated glowing one in the tannery’s small basement area. The power tools the Mariner is looking for is also located in the basement, inside a duffle bag. Beside the duffle bag, you can also find a red industrial trunk.
The tannery key can be used to open the locked office room in the ground floor and the safe inside. It can also unlock the alternate exit through the basement. As you exit the building, Machete Mike will come running to you and offer to buy the tools for 2000 caps. You can ask for more caps through speech checks or outright refuse the offer/ sell the tools. If you decide to keep your word, travel to Far Harbor and hand over the tools to the Mariner to complete the quest.