There are various sources of dam and lake data in South Africa. This dataset is an example from the comprehensive Water Resources of South Africa, 2012 Study (WR2012) website resource page.
Another option is the NGI Topographical Vector Data for various provinces which can be sourced from SASDI http://www.sasdi.net/search.aspx. This vector data includes a layer on hydrography (natural and man-made accumulations of water on the land surface).
The rivers in South Africa by drainage region (single KMZ 14Mb file) from the Department of Water and Sanitation (DWS) "Water quality data exploration tool" page. Covers rivers in South Africa across primary, secondary, tertiary and quarternary drainage regions. As they note, this is a snapshot of the DWS database and may lag behind the live database by several months.