Well Industry resource and others needed to built Ships and other things an be also added to others let say Japanese islands if they are separated. But it dosen't matter because mainland should be where you built ships and others. As for one city of Austria the thing happend to me that citizen of Trieste deposed my government to the Italy and I loose only coastal city. I'm playing at hardest level and maybe that is because of that. And in that case Hamburg should have access to Baltic and North sea. But still city itself could be that canal. Because If you loose your fleet you can't attack Denmark because its on Island. So I think that Denmark and Germany don't need to be separated.