Hello and good evening spuky,
if you can't sink the destroyer's there is the option to sink 2 other vessel's to complete the mission. It doesn't matter, what kind of ship's, even neutral, if they are moving in the circle of Danzig-Bay. If you can't sink any vessel, you can even move to Kiel Harbour. In this way observe, that your loose 1 crew point. Also observe not arrive before 4th sept in Kiel. If yes, you cannot complete your mission.
I admit, this part of the game is a well test for pacience. If you are ambitioness and without pacience ( like me ), then mayby it's better to start a new try.
Greeting's from Uli