You're kidding us. In WWI, deck gun kills were commonplace, but deck guns in WWII were nearly useless. U-boots usually did not get to go into position to attack lone vessels, and the amount of deck gun hits would be minimal simply because it could run away. Use against convoys is plain suicide. Moreover, U-boots are dreadful gun platforms, and vulnerable to small-arms fire, so getting into good range means a chance of the pressure hull being ruined, which would be an absolute killer. Obviously, it is you, who does not know a thing about submarine history.