I've survived Scapa Flow on my 2nd patrol - sunk the HMS Hood - but I doubt that I'm going back, as we barely made it and frankly, it was mostly a combination of luck and the incompetence of the British destroyer patrols. A few less drunken British destroyer captains and I would be on the bottom right now.
I'm currently going to attempt Loch Ewe (I heard some whisperings from our "intelligence" folks that the HMS Nelson may be in there). Honestly, it doesn't sound promising - according to this briefing sheet the place is laced full of nets and mines:
http://home.iprimus.com.au/waldingr/lochewe.htm
So I was wondering if you have any favorite harbors you like to raid? Maybe some that have good tonnage but aren't death traps?