Mnemba Atoll, off the northeast tip of Zanzibar, is the finest snorkelling and diving site in the archipelago — and consistently ranks among the best in East Africa. The atoll is a shallow ring of reef encircling a private island, declared a marine conservation area that strictly controls fishing and visitor numbers. The result is an underwater world of unusual richness: enormous schools of fish, healthy hard and soft corals, green turtles grazing on sea grass, reef sharks moving through the blue, and occasional whale sharks passing in season (November to February).
Day trips leave from Matemwe and Nungwi beaches, typically spending 2–3 hours at the atoll for two snorkelling stops. Dive operators in Stone Town and at the major beach resorts run PADI courses and guided dives; the atoll has a variety of sites suitable for both beginners and advanced divers, including drift dives along the outer wall. Visibility is consistently 20–30 metres in the dry season.
A word on practicalities: the atoll is a genuine marine protected area, and the rules are enforced. Standing on coral, touching marine life, or feeding fish is prohibited. Most reputable operators are strict about this. Pick your operator accordingly — cheaper boats are often larger and more crowded, which matters here.