If you only have one week, you're pretty much stuck with the Red Sea or something in the same time zone. Pemba is an alternative as mentioned. It was a few years since I was there, but I remember the diving to be very good. Extremely diverse marine life and coral growth, very good viz but extremely strong currents. And the diving was only good on incoming tide, which ment that you could only do two good dives a day. Otherwize it was very murky.
I also stayed on Manta Reef Lodge. It was a little run down, but not bad. And the location is very special. It is since taken over by a Swedish dive center and as I understand it, they have been investing some money in it. But even before (One Earth), the organization was good and safety oriented. Had own equipment, but rentals looked good as far as I could see. They had only one boat (one was in need of repairs) but we where only 5-6 guests, so it didn't matter.
Mozambique could be another alternative, but I think it would be very stressful since it takes some time to get from either Johannesburg, SA or Maputo to the dive areas like Tofo.
good luck