As with elsewhere in the Seychelles, diving and snorkelling are the main activities; the latter can often be combined with a trip on a glass-bottom boat. Parasailing is also a highlight here.
There's plenty of great diving within the bay of Beau Vallon, including a few wrecks, as well as some top-notch dive sites well outside the bay.
The bay of Beau Vallon hosts a few good snorkelling spots, especially along the rocky shore up the coast to North Point. It's also the main launching pad for boat excursions and snorkelling trips to Baie Ternay at the northwestern tip of the island, where the reefs are healthy and marine life plentiful. Full-day excursions include entry fees to the park, barbecue lunch and snorkelling gear. The best season is from April to October. A group of four to six people is required. Contact a recommended operator (or ask your hotel or guesthouse to do it for you).
At the Berjaya Resort, Underwater Centre/Dive Seychelles fits snorkelling (€25/45 for two/four hours) in during its dive outings to Îlot, Baie Ternay Marine National Park and the lighthouse. It also rents snorkelling gear (€10 per day).
Between August and October it's common to see whale sharks offshore from Mahé's north and west coasts. Underwater Centre/Dive Seychelles runs dedicated snorkelling trips focusing on whale sharks in September and October (€140 to €150), though don't expect to touch them – guidelines state you should keep at least 4m away. It also runs whale-shark monitoring programmes.