Lane, we are investigating Mauritius and Réunion too, have not been there yet, I have been researching for a potential trip there (most likely) in September 2027. Everything I think I know is just preliminary research, not based on actual experience there, so of course, big hunk of salt.
It's pretty clear to me that the two islands are very different. Mauritius is the more touristy (and more developed) of the two, the one with nice beaches and easy access to water sports, with better infrastructure. Réunion is more rough around the edges (and in the middle), gets fewer tourists, has less infrastructure, is more "wild." Réunion has a live volcano (which you can hike up) and very dramatic topography, while Mauritius is more like the typical tropical island that everyone dreams of (white sand, turquoise waters).
We are avid SCUBA divers (and snorkelers) so keenly interested in underwater stuff. That said, I'm finding very little about underwater Réunion - other than some fairly scary (and perhaps overblown?) shark stories. Everything I've read about shark attacks around Réunion sounds like over-the-top fear-mongering. For most divers, the presence of sharks is actually a sought-after feature, rather than something to be avoided; sharks are typically a very good indicator of the health of any marine environment, and divers usually love sharky places. I've swam with sharks before plenty of times, been bumped by playful ones a few times, so am not particularly skittish about them. That said, I'm not completely crazy either (really), and there are some places where you're asking for trouble by jumping in and trying to tickle the first shark that glides by. So maybe all the references I see about beeches with shark-netting being the only safe place to jump in the water are true (but I'm skeptical). From what I've read, it looks like there may not be great snorkeling from Réunion. Or maybe it's awesome and the shark attack stories are just to scare people away intentionally.
I'm finding a fair bit of info about SCUBA diving in Mauritius (but not around Réunion), and while it seems the water around Mauritius is clear and pretty, it does not sound like it's very fishy or the diving is outstanding there (that's from the perspective of SCUBA divers who often travel crazy distances to get to places that are very fishy).
There's very good diving (and snorkeling) in parts of Madagascar, as I'm sure you are aware. I've flirted with a trip there (the north end, around Nosy Be) where it seems the diving and snorkeling is outstanding. But (as you must certainly know) Madagascar is complicated and challenging. I've looked at a trip there many times but gave up when it seemed like I would need months to see it all. I had flights booked once but had to cancel them (still kick myself over that...). We will make that Madagascar trip one of these years. I'm envious and hope to hear about your trip after you get back. Good luck!
