See your GP for uptodate advice but it's likely to be: Pills for Malaria (Malaria is mainly confined to the eastern half of South Africa, especially on the lowveld (coastal plain)), Schistosomiasis (Bilharzia) (Bilharzia is also found mainly in the east but outbreaks do occur in other places, so you should always check with knowledgeable local people before drinking water or swimming in it)
Click here for Dept of Health
Click here for Lonely Planet Guide
Click here for Foreign Office advice
And remember about ice in your drinks, salads etc [all usually made/washed from tap water] stick to bottled water, & don't forget about AIDs; Happy Hols!!