AFAIK, the deepest dive of a type VII (that came back up) was U-331 in november 1941. After it sank HMS Barham, it dived to 250 meters (820 feet).
The Balao class could dive to 600 feet and one accidently dived to 1011 feet and survived, although the hull was so damaged, it was taken out of service.
http://www.usschopper.com/Chopper%20...e%20Report.htm