Christmas Island

There is very little dive infrastructure and the island does not need much: a small boat, a ramp at the cove, and most sites a short run along the sheltered stretch of coast near the settlement. Which side you dive is decided by the morning's swell rather than by a schedule, so the plan is made on the day. Shore diving is useful here rather than a consolation - walk in, cross the terrace and you are at the edge - which makes an afternoon easy to fill when the boat stays in.

Sites sort themselves by exposure more than by depth. The sheltered coves go in almost any conditions; long stretches of outer wall only open up on flat days; and the cavern systems cut back into the limestone want calm water at the entrance, proper training and, with most of the island inside a national park, permission to enter. Where fresh water seeps out of the rock into those caves it can blur the water into a halocline until you pass through it. Recreational profiles here run to about 40 m.

Two things bring divers this far. One is blue-water time off the wall, hanging with nothing underneath and watching what comes through. The other is fish: the island sits on the seam between the Indian and Pacific reef faunas, and species pairs that barely overlap anywhere else meet and hybridise here, which is why it draws fish watchers and ichthyologists specifically. Cave and cavern diving is a third draw, for the few who arrive with the training for it.

May to September is the driest and calmest stretch, and the rest of the calendar still dives well - what changes is how often the exposed coast is workable. Whale sharks are the one dated event, November to April. It suits divers who are comfortable in blue water and who accept a short site list, a thin flight schedule and plans that move with the swell. Non-divers are well served by the seabirds, the rainforest and the crabs.