Hi
Mozambique is a great choice, I went to Tofo (near Inhambane) a couple of years ago and still rave about it.
Not sure what you already know / have researched so apologies if Im stating the obvious
First up the location. Although Inhambane is the main town in the area (and has some nice colonial architecture) its 20km from the coast so most people dont stay there. Nearby Barra and Guinjata bay have small resorts which are supposedly nice though I have no personal experience of them. We stayed at Tofo which is a small village in a beautiful bay with a stunning beach. Its a really chilled out place and theres a fair range of reasonable budget accommodation. Dont expect the same quality of hotel/hostel as youd get in South Africa but what they lack in facilities they make up for in location. Theres a number of good bars and restaurants serving fantastic food (seafood and South African-esque Steaks). Best of all its all very cheap.
As for dive schools, Id highly recommend Tofo Scuba
http://www.tofoscuba.com who we dived with. The rental equipment is reasonable, theyre very safety conscious, and extremely knowledgeable about the area. (John discovered most of the dive sites in the Tofo area) Most importantly John and Nikki are extremely friendly and make you feel right at home. There are a couple of other dive schools in Tofo. One had just opened when we went so I couldnt comment, the other is Diversity Scuba who are based in town. We were originally going to dive with them but got a really bad vibe off them. Have heard from many others who had good experiences with them, and as we didnt try them I cant accurately comment on their service. Just one of those feelings we got. (I have no connection with Tofo Scuba by the way, I just thought they were very good)
Most importantly, the diving...
Im not sure what sort of diving youre into, but you should be aware of what you wont get in Tofo. There are no wrecks and its not a place to come if you want vast amounts of coral - There are a few sites (Sherwood Forest is one) which arent bad but if thats your bag Id look elsewhere.
However, what you will be coming for is plenty of big fish action. Particularly Mantas and Whale Sharks. Normally going to a site named Manta Reef seems to be the one place youll never see one. (The same with all those Shark Points/Reefs Ive been to maybe its just me!) But at Manta Reef it is pretty much guaranteed that youll see some. As they come into cleaning stations here, if you dont spook them you can spend a lot of time with them very close up absolutely mind-blowing. The other reason the area is famous is the number of Whalesharks that pass through. I think the main season is November to March but they get them year round so youve got a good chance of snorkelling with one.
Like you, we went in April and over 10 dives saw more than 20 Manta Rays but sadly the whalesharks eluded us (though they were some seen during our stay by others). Try and get to Manta Reef more than once, and also to The Office and Giants Castle. With a lot of friendly badgering we were able to persuade John to take us out to Amazon. Its a long way off shore and at the time only John knew about it (though I think some of the others have now found it) but it was absolutely stunning 3 huge Manta Rays, a Leopard shark, a few whitetips, two schools of Devil Rays (1 of about 40), and clouds and clouds of fish. Still one of my best dives ever. Ive been lucky to dive at a fair number of world class locations, and so far only Palau and Sipadan score as highly in my book. Youll have a great time.
Incidentally, be aware that the dive schools call some of the same dive sites by different names. The Tofo Scuba names seem to be the ones that are accepted and now appear in books on the area (The new LP/Pisces South Africa Diving Book has a Mozambique section in it). As for costs - We did a 10 dive package which was very cheap (prices are on the websites but all the operators charge about the same)
Hope this helps inform or inspire you. Im returning to Southern Africa in September and think Im going to succumb to the urge to return to Tofo for a weeks diving (the only reason Im hesitating is trying to persuade the soon-to-be-Mrs B that the accommodation wont be an issue.) Hopefully well get lucky with the Whalesharks this time and catch the migrating Humpbacks! Id really appreciate it if you could post a report of your trip when you get back as it would be good to know what has changed in the couple of years since we were there.
Andrew