A Fallout 4 modder has stumbled on a weapon that Bethesda, for whatever reason, didn't include in the game at launch.
The secret weapon found by xxdeathknight72xx in the PC version of the game is the Harpoon Gun. Like the Railway Rifle, it fires metal spikes at enemies. Judging by the gameplay video by Tyrannicon, it's just as effective, too:
There's no way to actually collect ammo for the Harpoon Gun, so you'll need to create some. You can do so by opening the console (press ~) and then typing "player.additem 000fe269 50."
You can see a faint trail of bubble coming from the Harpoon Gun so Bethesda presumably intended it for underwater use only. Maybe Bethesda planned out quests where players had to recover some valuable technology from the floor of the Massachusetts Bay. The Harpoon Gun would've been our way to fend off underwater creatures that likewise weren't added to the game.
It's already possible to explore the Bay by putting on some Power Armor. There are destroyed ships and other debris along the surface. However, there's not much to actually do down there other than simply observe it. It's a quest location in need of quests.
Maybe this undersea exploration will be a part of Fallout 4's DLC? Bethesda has said that they have multiple add-ons planned for the RPG. I wouldn't be shocked if at least one of them let us do some deep-sea diving. Why put all that effort into designing the Bay floor if you're not going to actually make use of it?
Even if Bethesda decided against that idea, I'm sure the modding community would run with it. After all, they've already got a fully completed weapon to use for any deep-sea mods they create.