If you want to dive, go to Egypt: Much cheaper and a lot more to see. Apparantly diving would also be very good on the neighbouring island of Pemba.
All dives are on Mnemba Atol. Not many choice in dive sites. Bad visibility (6-7 meters when I was there) A lot of sand and dead coral. But a great diversity of big fish and turtles. (I've seen dolphins during three dives, turtles, groupers, stone fish, scorpion fish, crocodile fish, leaf fish, ...)
About the dive center: not really professional. One of my buddies almost drowned, not because of the dive center, but they didn't even have oxygen on board to prevent decompression illness and the closest recompression chamber is in Kenia!!!