Depending on the year you can try the following areas
1939-1941: North and West of the British Isles and in the Irish sea (be careful though as it is infested with mines). Also the area around Gibraltar has a lot of traffic
1942-1944: North American Coast, Gulf of Mexico, The areas around Cuba and the surrounding islands have a lot of lone Tankers. You can also check the Area around Freetown and the tip of Africa. You will find Small Merchants and other ships in the area around Rio de Janeiro and in the Mar de la plata (Buenos Aires).
After 1944 don't even bother. Wherever you are there is an Allied destroyer and a couple of bombers looking for you. I usually spend the last months of the war in the relativelly safe waters of the Baltic Sea hunting Soviet shipping.
Good Luck
