Try Two Oceans (a guide to marine life of southern africa) by GM Branch, CL Griffiths, ML Branch & LE Beckley (struik publishers)
The only place I've seen it was at OR Thambo airport, think it was at the bookshop in the local departures down the escalators that I bought it.
It's 450 pages full color photographs with descriptions as well as a small map of SA next to each description showing where along the coast most likely to occur.
Paid R315.00 for it you can probably order it with shipping for about the same