I have heard diving in Cape Town is unreal. From diving with great white sharks to seeing cuttlefish,penguins or stingrays the diving seems awesome. It is one of the places I plan to go to dive as soon as possible. But one downside that you have to prepare for is that the water is pretty cold in October and November, the water temp is mid to low 60s so be sure to bring a thick wetsuit or a dry suit if you go in October or November.
