i remember years ago in the stock version i downed a BB with the deck gun...
i was in a similar situation, ran across task force, hit the BB with my last 2 torpedoes, evaded destroyers they ran away after a while but the BB was stopped listing to port badly. so i surfaced about 100 meters off the starboard side and just used every shell i had on the BB as he couldn't bring his guns low enough to shoot me.
eventually a few fires broke out, i think it took like an hour, but eventually he went down
i wouldnt suggest this in GWX