Hi,
Well, its winter here so you can expect fewer bums on dive boats which can be a big plus depending on how you look at it.
But in all honesty, the coastal areas here dont have a serious winter, it may go down to a (considered real cold here) 14 degrees centigrade at night but expect the days to warm up to 25-28 degrees centigrade.
The diving here is generally much better in winter as the seas are calmer and the water usually cleaner.
Off the Aliwal Shoal (Natal) in June and July you can expect to dive with lots of Sharks as they follow the sardines up the coast (you may even get to dive with the sardine shoals) - a 5mm wetsuit is fine for this area.
In Mozambique you can expect lots of great coral reef dives and the occasional Basking shark and / or whale (various types) as they also move to warmer waters, a 3mm - 5mm suit is fine here.
If you intend to dive in Cape Town there are various Great White dives which are very active at this time - bring a 7mm wetsuit for this dive plus hood and gloves.
So in all, its actually the best dive time, the only cold place will be Johannesburg where you may want to do some touring (no diving there) where you can expect 3-5 degrees centigrade at night warming up to a balmy 20 in the day.
Let me know if you need more specific info let me know, happy to oblige.