Meh!, I haven't used the Deck Gun in this campaign yet and we're in January 1945, the last time I had to use one was when we had to try and fight our way home as the sub was too badly damaged to submerge.... it did not end well for us.
So you're not into realism then Col, how about gaming the game, otherwise known as manipulating the files on the fly.
Col you need that SHARPSHOOTER chap from the special abilities file on your boat and, as a fellow submariner I feel obliged to help you out with that. You just let me know and I'll tell you how to put him aboard your boat in your current campaign, Meh!, your next mission I meant to say.