Eckhard,
Kob in the area are not seen to often (by me or any of the guys I dive with). In the 30 odd years I hacve been going to mdumbi to my place, I have not seen one shot. For some unknown reason they are also not caught deepsea there too often either.
That said, the untata river is excelent for kob particulatrly in the mouth area. The beach from the river mouth towards the anchorage hotel also good(fishing wise)
They are also caught from time to time in the channel fromt he boat launch, but not too often.
Fishing wuise, harris's gully produces the odd one and is dibvable, Bird rock produces good kob fishing and is very divable.
The bay in front of the tshani cottages (2nd bay past bird rock, with pink double story cottage just off the beach also good for kob. Depending on sea, rip etc, either the left or right (looking seaward) channel will produce. My boet missed about a 20kg kob right in the RH channel a year or so back, but its a luck to see them as you either entering or exiting.
The left hand channel not really divable as not much out there bar sand. maybe giving a few divies into a hole muight produce as this is where they are caught, but its usually churned up.
The rocks to the right of the cottages may produce but usually catch reef fish there.
I hope you come right, but experience doesnt tell me its great kob area.
in order of prefenence, Bird rock, harris,s and mdumbi around to harris's. I guess you staying at back packers. Speak to Lusanda. he should know if they being caught anywhere and this will guide you. he a good bloke. His mother and boets work for me.