Historically the U-boat personnel was part of the Kriegsmarine, the German Navy. They did not have a direct connection with the National Socialist party. In fact, because the U-boat men were isolated, the Nazis wanted to make sure that they were still properly indoctrinated and sometimes sent Nazi agents on missions to rat people who talked badly about the party. If anything, the Nazis were a menace to their own military too, which could be an interesting gameplay twist, having to deal with the stress added on the crew by the presence of a Nazi agent on board.
It’s an interesting historical aspect of the war and I don’t plan to shy on it. I believe that the only legal restriction is the display of the swastika in certain countries, but I have no reason to show any so I don’t think this will be an issue.