I just returned from Mauritius, I wasn't impressed by either the diving or the snorkelling, it was pretty but I wouldn't make the trip again.
Loads of Sargent Majors, lots of black tangs, lots of Picasso triggers, and tons of eels, didn't see any sharks or other big game fish, a little disappointing.
I heard the Maldives is the better stop, which is just where I will be heading next year.