The diving around Mahé sits in a category of its own, largely because the geology is so unusual. Instead of the wall-and-channel structures you find across much of the Indo-Pacific, you're weaving between enormous granite boulders that tumble down from three or four metres to around 25–30 metres at the deeper sites.
The rock formations create swim-throughs, undercuts, and shadowed corridors where nurse sharks rest and large moray eels hang in plain sight. Visibility is generally good — 15 to 25 metres on a settled day — though the inner islands can cloud up quickly after rain or when current patterns shift.
Currents here are mild compared to, say, the Maldives or Komodo, which makes it comfortable territory for Open Water divers. That said, conditions vary considerably between the northwest and southeast monsoon seasons, and a handful of sites become genuinely tricky when swell picks up.
Green and hawksbill turtles are almost guaranteed at sites like Shark Bank and Beau Vallon, and if you're in the water between August and November, whale shark encounters are possible offshore. Endemic species including the Seychelles anemonefish appear on shallower sections, making snorkelling from shore worthwhile rather than just a consolation for non-divers.
Reef condition is mixed and worth being honest about. Bleaching events in 1998 and again in 2016 hit hard, and some sections look scrubby and diminished. Recovery is visible in places, particularly around protected marine parks, but you'll also encounter areas of notable coral loss. It's beautiful, just not pristine in the way the brochures suggest.
Day-boat operators are plentiful around Mahé and Praslin; liveaboards exist but are less common than in other Indian Ocean destinations. Open Water certification is sufficient for most sites.