Fallout 4’s weapons already veer towards the crazy side- there’s an impressive array of them in the game to begin with, and with the added variation that mods and customizations can bring to the table, there is no shortage of instruments of murder.
But arguably the very best weapon of them all is hidden somewhere in the game’s code.
Modder xxdeathknight72xx has discovered hidden in the game’s files a big harpoon gun, complete with an underwater animation when you fire it. The animation is fairly elaborate and complete, suggesting either some kind of an underwater quest that will be made available later on, presumably as DLC, or that there was one planned for the main game at some point in time before it was scrapped (maybe the Glowing Sea quest?)
But you can still have it, and you can have it now- you can download the mod here. The harpoon gun takes the place of the railway rifle in your inventory, and since it doesn’t technically exist in the game world any more, you’ll have to use console commands to generate ammo whenever you need it.