For one thing, divers don’t mind the rain… they’re going to get wet anyway! But, surprisingly, winter diving in Cape Town is often better than summer diving. At least part of this has to do with the fact that conditions are ideal for diving False Bay (my favourite side of the peninsula), where the water is warmer than the Atlantic. The prevailing winds are northwesterly, and this flattens the sea in False Bay. The bay is also protected from the winds to some extent by the mountains around it. The result is that the visibility in winter is magnificent. The water is a bit colder (more towards the 12 degree end of the range), but because the air is cooler than it is in summer, you won’t actually feel much of a difference, and in fact it often feels relatively warm. Cape divers will tell you that winter is the best time for diving here – don’t miss it!
(This information also appears on my website, here.)