The best snorkelling reachable from the shore: a protected lagoon near the airport with corals metres from the beach and a public beach behind it.

Most Mauritian lagoons are sand and seagrass. Blue Bay is coral, and it starts a few strokes from the beach. Fish that elsewhere need a boat trip are here in two metres of water. It is a marine park, so the reef is in better shape than anywhere else on the coast.
Free beach; glass-bottom boat trips are paid and leave from the beach. Snorkel from the eastern end of the beach towards the marker buoys. No standing on coral, no feeding fish. Ten minutes from the airport, so it works as a first or last day.