Mnemba is a small private island off the north-east corner of Unguja, ringed by a marine conservation area with the healthiest coral and the clearest water on the island. The island itself is a private resort and closed to visitors, but the reef around it is open, and it is the best snorkelling ground in Zanzibar.
Boats leave from the beach at Matemwe or Kiwengwa in the morning, when the water is calmest. On the way out the crew look for the resident pods of bottlenose and spinner dolphins. Sightings are common but never guaranteed, and the crew keep a respectful distance and follow local guidelines on how to enter the water.
At the atoll you snorkel over the coral gardens. The reef here drops away gently, so the shallows suit beginners while the edge rewards confident swimmers. Angelfish, parrotfish, moray eels and turtles are all seen regularly. Gear and life jackets are provided.
At low tide the boat will also stop at a sandbank near the atoll before returning. Fruit and soft drinks are served on board, and you are back at your hotel in time for lunch.