There's a small historical error in Baltic Operations campaign - Danzig is treated as a Polish city, while in reality it was a
free city state, inhabited mostly by Germans and from 1936 onwards ruled by Nazi party. Only part of Danzig that was under Polish military control was a small Westerplatte peninsula, which housed a Polish military transit depot. It was this depot that's been a target of opening salvos of the war fired from Schleswig Holstein battleship.
In terms of our game, while being shelled from shore defences near Danzig can be somewhat historically correct (Westerplatte garrison had 2 37mm and one 75 mm guns, which theoretically could target a careless U-Boat), Polish ships docking inside Danzig port are not - the port was under German control from the start of the war.