Since you mention snorkelling as a major attraction of the trip, you might want to bear in mind that in many parts of the Indian Ocean the coral has suffered badly from overly warm ocean temperatures. My brother was in Seychelles recently and was shocked at the degradation of the coral, and I hear the Maldives have also suffered; not sure whether this also applies to Mauritius. However, if considering Seychelles I would also consider Lemuria resort - looks fabulous, and I've read good things about it.

In the Pacific, there is great snorkelling in parts of Fiji (Taveuni, southern Vanua Levu) but this might fail on the 5* front. On the other hand, French Polynesia has lots of 5* accommodation but I'm not sure whether the snorkelling measures up.