In a situation like that I would try bottoming the boat. Even in shallow waters (as little as 40m) it can confuse the destroyers ASDIC enough so they cannot hit you dead on. When I do 2 knts I only float 1m above set depth...
If you want to bottom the boat all you gotta do is take it right down to the sea floor slow it down to about 2knts or so, then hit D just above the floor and the boat will do down and stay down. On the bottom they won't be able to pinpoint your exact position.