Try these links. They have some tips that might help you out:
http://www.subsim.com/radioroom/showthread.php?t=96843
http://www.subsim.com/radioroom/showthread.php?t=105210
The "walls" are actually anti-submarine nets and were laid to prevent people like you and me from messing up the RN's day.
You said you are no-GWX so I assume you have no mods? If no mods then I don't think the info below will help.
The entrance you should always use for Scapa is the same as Prien. go to the eastern side and manouver around the block ships through one of the narrow channels taht lead into the harbour.