Hey Jon
If you just installed GWX, here's one tip;
Stay away from the channel. When you do meet a ship, big chance it's gonna be looking for you, and dropping 130 kilos of good news on top of you, in the form of depth charges. Without the ability to go much deeper then periscope depth.......
If you're looking for lots of ships, look at the convoy map
http://etnies.muskatli.hu/sh/faq/SH3_MAP.jpg
Usually, what I do
(Departing from Lorient), I go up north, join the convoy route that passes between Brest and Bristol, and go west, and hang around the area where the 2 convoys from the south cross the one going to Halifax. Lots, and lots of ships. I usually run out of torpedo's within 7-10 days of my patrol start