I've had a few friends head south from Kenya to South Africa and only said good things. I also had a friend who volunteered in Tanzania at an orphanage for a few months and raved about the country and the people.

I'm not a diver myself, but I can imagine the clear blue seas would be an excellent place to explore, plus it's not massively touristy so it would be pretty much untouched, especially compared to the more well known diving places like in Thailand and Australia