Just set up the Hail Mary full of grace scenario where the allies have one engineer already emplaced acting as a ferry.
I am now unsure whether the german player can attack this unit or not. If I understand it correctly sea lines prohibit attacks, so does this mean it can't be attacked by normal units? What about artillery? And if artillery creates an DG result, does the unit need to retreat from the full river hex, since DGed engineers can't act as ferries? And in that case, can I choose on which side of the river it retreats to?