My wife and I went snorkling in SA just south of the Mozambique boarder and it was beautiful... On the two seater flight up to the resort (RockTail Bay, great place) we saw a ton of tiger sharks (swimming not so far off shore) and a few whale sharks.
We were there around the middle of Oct which is when the turtles nest (leatherbacks). Although we saw their tracks on the beach from the plane we weren't lucky enough to see an actual leather back. We did, however, see a pod of whales from the shore, and they weren't very far off shore.
As I was very intrigued by the area (Indian Ocean, I've heard wild things about Madagascar too) I made it a point to pick some brains if I got the opportunity. Subsequently I have met three groups who have been to various places: Two were older/wealthier Americans (met in Hoboken, one works at the dive shop there) One dove out of Cape Town for the great whites another dove out of Durban also cage diving with great whites. Neither of these trips seemed too appealing to me as they were all about the shark and nothing else.
But, I met a group (a family and two girls they happen to meet up with) in Phi Phi Thailand who were from SA. They all raved about the 'untouched' diving in northern SA and Mozambique (the family hadn't dove in Moz because of safety concerns but had been regalled with stories). I spent a couple of days and dinners with the two girls who had done very extensive diving around the world including a substantial amount in eastern Africa.. all up the eastern coast of Africa (there were some big bucks in their background... private boats, three week excursions here there and everywhere, not just diving... skiing, sailing etc.) They raved about the virgin diving in the area you are speaking about. The big problem if you weren't 'connected' (and they were) was finding sites or guides who know the area.
I also dove with a DM in Mexico who was from Kenya and had also dove in Moz. Although he only had about 70 dives under his belt at the time he thought the diving in Moz was fantastic but the industry wasn't set up for tourism. His father and brother were leading an effort to sink a wreck off of Kenya and meeting stiff resistance. He was in Mexico getting various certifications (cave) and learning the business from a Spanish operator that had three or four operations on the Yucatan and several others world wide. I got the impression that his family has some money and were venturing into opening up a business in eastern Africa and he was learning the ropes. He spoke about 'virgin' diving from Mozambique right up through Sudan. The reason I bring it up is he confirmed what the girls had said (and the girls had dove all over... they spoke about 3 and 4 week excursions through the Pacific, Malaysia, Indonesia... neither had ever worked a day in their lives). Of course they were both South African and may have been prejudiced towards the area.