This new Exploit in Fallout 4: Far Harbor let's you have an infinite amount of harpoons and money. With this glitch you can sell your harpoons and get unlimited amounts of bottle caps if you're patient.
Before you start the Harpoon exploit in Fallout 4: Far Harbor, make sure that you have at least 20 harpoons on you and a Harpoon gun.
You can get both from the Far Harbor vendor.
Now got to a wall and shoot 20 harpoons in the exact same spot. After that you can pick them up infinitely. It requires a bit of patience but you can basically stock up on thousands of harpoons that you can sell for bottle caps.
You can sell these to vendors and pick them clean with your unlimited money.