We did this about 4 years ago. Most of this happens in Gansbaii (Hansbuy). This is East of CPT on the Garden Route, it is worth spending a week exploring the South coast's Garden Route, so many things to see and do.
No need to know diving since the cages just sit at the surface. You just snorkel.
We had breakfast prior to the day which I think was used by most people to help chum for shark.
They used a "fake" seal to attract a shark as well as chum the water. The are pleanty of real seals around. A shark came by quickly then disappeared. After about 2 hours and getting bored my wife and I got into the "Tomato cage" just to say we did it. Just then then shark returned. We got some great photos, unfortunately on film, not digital so they are harder share.
The caveats are you are in a wetsuit and the water is cold. If you are prone to seasickness take something prior.
The night before the dive company set us up with a room overlooking the water and we had Southern Right Whales playing about 100 yrds away while we had cocktails on our balcony.
If you drive out that way you can stop at Hermanous and have lunch in a restaurant cut into a bluff with the whales cruising around nearby (this isn't Disney so they might not be there).
Remember right now it is close to the end of Winter, so plan your trip accordingly. Animal watching is best in the Fall/Winter when there is less foliage. CPT can get very damp and cool in the Winter. It isn't too far South, but the cold water effects it.
I am thinking I need to digitize this album.
SA is a great place to spend a few weeks.