A modder rooting through Fallout 4's assets found an unused weapon whose unusual animation hints at some kind of underwater level in the game, either planned and scrapped or yet to come in DLC.
Modder xxdeathknight72xx found this harpoon gun, which replaces the Railway Gun if one has that in his or her arsenal. Of course, a spear gun would be a practical weapon on the surface, especially given Fallout 4's improvised, modded mien. It's the bubble animation surrounding the kills that indicates Bethesda Game Studios may have had bigger plans for submarine combat.
Fallout: New Vegas had an underwater mission, though combat did not feature in it. On the surface, YouTuber Tyrannicon here shows it pinning a raider's noggin to the wall, sort of like Rambo did with the explosive arrow in First Blood Part 2. Information on how to bring it into the game (PC only) is available here.