Johan, your boat will have to be surveyed and a boayancy certificate obtained. You will have to have a vessel over 15ft6 to exit the harbour mouth, you will have to obtain a vhf radio operators licence.
When you join any of the angling clubs in Durban harbour you will be issued with a operators permit to fish the harbour as well.
The clubs you can enquire with include -- Bobbies angling club, Fynnlands angling club, Neptunes angling club, pompano angling club, Bluff yatch club (They have a motor vessels section to)
Now in terms of facilities on offer at these clubs Fynnlands and BYC are the best.
Google fynnlands angling club and check there web page