[Bananadine]

But that still leaves the question of how to make the two weapons in the first place. I don't know how to declare in the mod's specification file that a missile weapon is supposed to work underwater.

A weapon-copying command would suffice, since the Ethereal Crossbow (and some non-forged weapons, such as illithids' mind blast I think) have the property I need. But I don't know of any such command.