The Tanzanian coast has warm water between 25 and 29 degrees all year, extensive fringing reef and three distinct diving areas. Coral bleaching in 1998 and again in 2016 did real damage, and recovery varies by site, but the fish life remains excellent.
Mnemba
A small private island off the north-east corner of Zanzibar, surrounded by a marine conservation area. It is the most accessible diving in the country, with easy sites of 8 to 30 m, good visibility and reliable turtles, moray eels, reef sharks and dolphins. It is also the busiest, and boat traffic at the main snorkelling site is heavy in high season. Go early.
Mafia Island
South of Zanzibar and completely separate from it, Mafia has the largest marine park in the Indian Ocean and a fraction of the visitors. The reason to come is Chole Bay and the whale sharks: between October and March a resident aggregation feeds in the plankton-rich water off Kilindoni, and swimming with them is possible on snorkel with a licensed operator. Unlike most whale shark sites these animals are present for months rather than passing through.
Pemba
The northern island of the archipelago is greener, hillier and much less developed than Unguja. The Pemba Channel drops to 800 m close to shore and produces steep wall diving with strong currents and large pelagics, generally rated the most demanding and most rewarding diving in Tanzania. Misali Island on the west side has shallower reef and nesting turtles.
Seasons
Diving is possible year round. Visibility is best from October to March, often 20 to 30 m. The long rains in April and May reduce visibility and many operators close. June to September brings the kusi wind from the south, which makes the east coast choppy but keeps the north and west diveable. Whale sharks off Mafia are October to March; humpback whales pass the coast between July and September.
Turtles
Green and hawksbill turtles nest on several beaches, with Mafia, Misali and parts of the Zanzibar coast being the main sites. Nesting runs roughly from February to July and hatching some sixty days later. Conservation projects at Nungwi and on Mafia allow visitors to see releases; avoid the enclosures that keep turtles in captive pools as an attraction.
Practicalities
Bring your certification card and a logbook. Nitrox is available on Zanzibar and Mafia but not everywhere. The nearest recompression chamber is in Zanzibar town, so insurance covering diving evacuation matters. Reef-safe sunscreen is increasingly required by operators and is not always available locally.