Mayotte

A diving day here starts with a crossing. The handful of centres work out of Mamoudzou and the beaches down the west and south coasts, and because the barrier reef sits well offshore, boats spend real time on the water before the first dive: sheltered running inside the lagoon, then swell once you reach the reef line. Trips are usually two dives with the interval spent anchored somewhere calm, and pass dives are timed to the tide, since the flood pushes ocean water in while the ebb drains lagoon silt out through the same gap.

The reef structure is unusual. In places Mayotte carries a double barrier, an outer reef with a second one behind it, and the water inside is a broad plain of seagrass, sand and scattered coral heads rather than continuous reef. That gives three quite different dives: quiet inner-lagoon bommies where the current is negligible and the work is macro, the outer drop-off falling away on the ocean side, and the passes, where the whole lagoon's water has to squeeze past you.

Mantas are why most people come, and because they are resident rather than a seasonal aggregation you are choosing weather instead of chasing an event, though numbers on any given day still vary. The other appeal is quiet: sites get dived by a few boats rather than a fleet, and you are rarely sharing one. It suits divers comfortable with a French-speaking, expensive and thinly served island, who will fill the rest of the trip with seagrass snorkelling over grazing green turtles and, in the right months, whale song audible on a dive.