In Plettenberg Bay you get diving sites like Groot Bank, Redbait Reef, Deep Blinders, Shallow Blinders, Dolphin Column, Jacob's Poort and Whale Rock, as well as the MFV Athina wreck (1967).
At the Tsitsikamma Marine Park there are trails for scuba-diving and snorkelling, but conditions are rough most of the time and it could easily become dangerous. Having been a marine reservation for so long, however, allows you to see marine creatures not often seen.
In Knysna there's the Dagleish Bank, East Cape Reef, Bruce's Bank and the Pinnacle, with the wrecks The Fairholme (1888), The Paquita (1903), and the Phantom wreck.
At Thesen's Jetty in the Lagoon in town you will find a junkyard, but underneath the jetty itself, fascinating little seahorses are to be found.
And in Mossel Bay there is the reef at Klein Brak River Mouth, Stingray Reef, Mitch's Reef, the Santos Reef, Dolosse and the shallow and deep blinders of Windvogel Reef.
Care should be taken, however, when diving near the Seal Island, as fatal shark attacks could occur. The safe way to observe this area, is in a shark cage, from where you can observe the sealife and the sharks going after the seals.
See Encounter Magazine's comprehensive guide on Scuba Diving
in South Africa.