Mafia Island

Diving here is governed by the tide. The centres sit on Chole Bay, a wide sheltered bay on the island's eastern side whose entrance shallows enough that boats can only cross it, or dive the pass itself, on the right stage of water. Schedules shift day to day as a result. The rides are short: small open boats leave from the beach in front of the lodges, run to a site inside the bay or just beyond it, and are back before the afternoon wind gets up.

Inside the bay the diving is sheltered coral garden, hard coral heads and bommies standing on sand, with undemanding conditions and heavy fish density, which is why it keeps working when the sea outside does not. The pass is the opposite: a channel where the bay drains and fills, dived on moving water with the current doing the work. Outside, the reefs open into slopes and a rock wall, deeper and more exposed, and they want a settled sea.

The whale sharks are a separate excursion on the other side of the island, reached by road transfer to Kilindoni and then a boat that searches for animals feeding at the surface. Those encounters are on snorkel, in shallow and often plankton-thick water, and finding them is most of the exercise. Beyond that, this is an unhurried, lodge-based island with a handful of operators and no liveaboard traffic, which suits divers content to plan a week around tide and weather.