One of the most fun things about a new Bethesda RPG is waiting for the community to rifle through its files for concepts abandoned during development. Modder xxdeathknight72xx has been on the task and uncovered a bonanza: an underwater harpoon gun in Fallout 4.
As the video by YouTuber Tyrannicon demonstrates below, the gun works fine out of water, though if you look closely you'll see bubble trails. That seems to prove that at some point, Bethesda intended for underwater combat to feature in the Commonwealth. I'm kinda glad they left it out, to be honest. Water scares me.
Still, I wouldn't be surprised if forthcoming DLC packs feature some element of underwater exploration. The gun replaces the railway rifle, and can be grabbed over here. If you want to see some of the coolest and/or strangest sites witnessed in the wasteland so far, James Davenport has compiled them over here, while the best Fallout 4 mods are conveniently presented here.