An unused weapon was discovered by a Fallout 4 modder recently, UNDERWATER. This harpoon gun was speculated to be used as a possible DLC weapon for a new mission of some sort. The death animation for an enemy when shot with the weapon would show surrounding bubbles which indicates that the weapon was supposed to be used for underwater combat. Also, the Harpoon gun replaces the Railway Gun if placed in your inventory. take a look for yourselves:
What do you guys think? Underwater combat sounds awesome as a DLC!