Try Scapa Flow and you're up for a real challenge. Getting in is fairly easy (approach from the east and you'll get a good look on the barricade ships
), but getting out is so darned hard.
It's got a harbour in the northern end of the bay (don't know if it used to, but it does now!), but it's a long way out from there, with plenty of ships and shallow waters.
One mistake I did was to assume that once I sunk a ship, the enemy knew where I was, due to their numbers and the lack of depth. So after firing my torpedoes, I set throttle to flank and tried to dash out, since "they knew where I was anyway". Bad idea. Stay on Silent Running until detected,
then make a run for it!