The Wolfpacks in GWX have surfaced uboats shooting with the deck gun. In Sergbuto's Mod they run submerged at periscope depth and shoot "virtual" torpedoes which are implemented in the mod.
If you're using GWX it is not enough to simply install Sergbuto's Mod, because his uboats are not scripted into the campaign and simply wouldn't work, so you have to implement them by opening the campaign_RND.mis file with a text editor and substituting the original GWX uboats (NSS_uboat7c41) with Sergbuto's (NSS_uboat7a). That way you get GWX with submerged wolfpacks.
(Make a backup of the campaign file first, and save your career while you're in the harbor!)