Underwater firearm seems pretty useless since there's only a handful in total, mostly either obscure prototypes or equally obscure Russian special-issue weapons; Wikipedia lists eight including the cancelled Lancejet and AAI's underwater revolver than never got past prototype. They don't tend to appear in anything since Hollywood has a strong preference for spearguns, and I really can't see the use of a category with only one gun in it.
UGBL is potentially useful. The way I use categories for guns, if I see a weapon I don't recognise I'll click around the entries in the cat looking at the page images before searching offsite. Obviously, if I'm looking at an underbarrel launcher I'm not likely to want to search through a list full of standalones.
Also, this still needs killing:
http://www.imfdb.org/wiki/Welrod_Pistol