If you want to play historically U-Boats out of Bergen were assigned to the Arctic sector north of where the grid map ends in game. You can find some juicy convoys up there on the Liverpool to Murmansk convoy route.
Every time I have been up there has been very rewarding. Very little to no air cover tons of convoys with juicy targets, tankers, large mercs you name it. Some convoys are escorted by careers so make sure you don't get nailed by those avenger dive bombers.
Be extra careful when snorkeling do not exceed 64 TC because they do spot you and they do bomb you I have experienced it first hand
