I recently did a Scapa Flow incursion, followed the historic path that Prien took. Fairly easy to get in if you catch the weather at the right time (good visibility and calm enough you wont get pounded on the bottom). I got stuck outside the harbor for 3 days, had to avoid ASW patrols for that period in 15m water. Little stressful.
Then there wasn't anything worth shooting in there.
Just saying this as a tip for anybody else who want's to try an incursion. It's built into GWX to leave that path open, just do a little research and recon. I found it lots of fun to do my own recon and such, just like Prien.