How much of time is 'a lot of time'? Sometimes you just need to have some patience. Even coastal waters, while they have a lot of traffic, still don't have that much traffic and you can't be everywhere so some ships will slip through.
In addition to the convoy map, maybe check out the campaign RND layer in the sh3 mission editor? This can give you some info about the disposition of the ships the game throws at you. Looking at AN 18 and AN 26, it seems like you'd expect maybe 3-5 single merchants passing by each day, not a lot of convoys, and plenty of warship patrols. Makes sense - that area is sort of the RN's backyard grill patio in 1939 and you're the annoying wasp that wants to have a taste of the steak that's being marinaded. (except that the steak is 2800 tonnes of bauxite and the marinade is aviation gasoline)
And remember that as a WW2 uboat your main means of movement is on the surface. Stay on the surface for as long as you can, that's where you are fast, your diesel oil is plentiful and your battery power is scarce. Of course, as the war goes on this modus operandi becomes more and more difficult, but that's not something you should worry about in 1939.
If you really are desperate for enemy merchants you can try visiting a harbour. But that's not exactly a walk in the park, not even in 1939, not with GWX or Onealex at least. In vanilla you can, but that's another story, basically another game.
Oh, and hydrophone is better for detection than visual, so remember to do hydro checks when hunting. You can pick up sounds from further away than the hydrophonist, but I tend to not use that, as I find it tedious and the hydrophonist's range is pretty realistic, IIRC, as a max range for the GHG/KDB.
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