before 1941 it's very variable, if you don't make stupid mistakes you'll usually get away
early 1942 is ocassionally challenging if you run into a nasty Tommy with some skills
on towards and into
1943 it gets real rough, especially on approaching convoys because of Radar and such, you'll also see DEs with more and more DC launchers as well as the first hedgehogs
It'll also get a whole lot nastier Plane-wise
1944 is just plain evil, even with a type XXI below 250 meters it's almost impossible to shake the ASDIC once they get a fix on you, and there's usually three of them hunting you
1945 is not all that much harder than 44, the battle of the Atlantic is lost by then, the best you can hope for is to find some (relatively) unprotected shipping North of the British Isles or out in the Indian Ocean
Don't even get me started on the Black Sea in 1944
