Uuups, I did it again...
I came back to Dunkirk on February 1940 in the same boat. Everything was all the same. Everything. Same Polish destroyers, same French troop transporter, British large tanker and so on. No patrol crafts around the port.
I went in, spent half a day inside waiting for the French troop transporter to sink (sadly, she didn't and I had no more torpedos) and left. DD's were pinging at me like a bunch of crazies but they couldn't move. When I got to the surface for a moment they immediately opened fire from all guns, so they weren't out of service at all. Not a single airplane appeared nor any warship.
Ships in the port of Dunkirk are the easiest prey in the game I think, and exploiting this is cheating for sure.