The best known surfing spots are around Cape Town, but there are loads of great, virtually unsurfed waves. The many backpackers hostels around the region are geared up for board rental, escorted surfaris and surfing lessons, and there is a dedicated surf school in Cape Town. There are awesome windsurfing and kitesurfing spots. Langebaan, near Cape Town, is internationally recognised to be one of the top boardsailing venues in the world - and there are others.
A more accessible way to experience the sea is in a specially designed sea kayak. There are escorted trips around Cape Town, Hermanus, Knysna and Plett. For a bit more of a challenge, try surfskiing - it's becoming a rather trendy pastime and can be very competitive. Surfskis are really fun boats to paddle - fast, responsive and with the approximate stability of a razor blade balanced on its edge. There is a whole series of races throughout the year all along the coast.
Sailing
The South African coastline is one of the most challenging in the world to sail, with few harbours, stormy conditions and, often, rough seas. So it's not a good bareboat option. However, it's a great place to learn to sail - there are sailing schools in Cape Town - and South Africa features in all the major round-the-world races. There are loads of local races, too, and the Cape to Rio is one of the most accessible ocean crossing races in the world, attracting a large cruising contingent.
Diving
If you need any convincing about the seriousness of the coast, you need look no further than the many historical and recent shipwrecks. Of course, these were all tragedies but still good news for local divers. There is more to offer than these poignant sites, though. The underwater environment is absolutely beautiful and varied and there are lots of wonderful diving sites. South Africa has an enormously long coastline ranging from about 35°S to 27°S, which isn't quite within the usual range of tropical diving. However, the Mozambique Current which flows down the East Coast brings warm, tropical water with it, and at Sodwana Bay there are the most southerly coral reefs in the world. Of course, they have the full complement of pretty colourful fish and some great nudibranchs, including the outrageous Spanish dancer (Hexabranchus sanguineas). Whale sharks, turtles, dolphins and ragged tooth sharks (Carcharias taurus) are often seen in specific places. And then, as you head down the coast, the underwater faunal and floral assemblage changes gradually until, once you've reached Cape Town, you're diving in chilly but beautiful kelp forests. These, too, are unique. There are three major types, or genera, of kelp and it is only off a short portion of the Western Cape coast that they all grow together.
If you've always shunned cold water diving, consider it. You do have to dress up in a great thick wetsuit with constraining hoodie and gloves but it's worth it. Diving in kelp is like walking in a forest. You float beneath the canopy and admire the surprisingly colourful reef life. Off Cape Town, divers regularly see anemones in colours ranging from electric blue or deep red to pale pink, nudibranchs of almost every colour you can imagine and a whole range of small creatures in and around the bright orange and sulphur yellow sponges. There are dive schools in almost every centre.