If you are wanting to exit the harbour mouth your boat must be a min size of 15ft6 with 2 motors, you need a VHF license, Durban operators certificate (DOC), your boat must have a valid COF and current boayancy certificate.
To fish in the harbour (Catagory R) there is no minimum size for a boat. It just needs to have a valid boayancy certificate and COF (Certificate of fitness) and the required safety equipment for Cat R. You will also need a current Portnet Harbour permit.
Your best bet is to join a harbour fishing club