You might want to give the War Ace Campaign mod a try. It may not be as historically accurate as GWX, but I've never failed to come across some kind of action (except perhaps way out in the middle of the Atlantic out of the shipping lanes).
For example, while heading home passing through the English channel I encountered a fishing trawler. Small fry, but I figured for the fun of it to surface and sink it with the deck gun. The trawler was just about in range when two Brit aircraft showed up! So I crash dived and just as the Brit planes flew on a couple Luffewaffe aircraft showed up and strafed the trawler! :p
I thought since friendlies were in the sky to surface and finish off the trawler, but as soon as I did the Brit aircraft showed up again! So it was back under the surface. I decided this was too much trouble for a paltry trawler and headed back on course to Brest. This was after a satisfying patrol of sinking, among other ships, a pyro that put on a lovely show when it exploded and an ore carrier.
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