Pretty new to career mode within SH3, but one area i've found is peachy spot just SW of holyhead. I'm running a IXB out of Lorient in Oct 1940 and am on my 2nd Career. On my first I hit a huge convoy there and bagged four or five ships before running out of eel's and getting D/C'd to my grave!
In my current career I again hit a convoy SW of holyhead just the other night, so I wld suggest giving the irish sea a try. Also keep an eye on where you are on the map cause the darker colors highlight the deeper depths.