Cocos Island

Everything runs off the mother ship. Divers ride out to the sites in pangas, roll in negative and drop straight down before the surface current takes hold, then regroup on the rock below. Most days hold three or four dives, with the tenders circling afterwards to collect surface markers — carrying and deploying one is not optional in open-ocean swell. Between dives there is nowhere much to go, since landings are at the rangers' discretion, so for the length of the trip the boat is the whole world.

The sites split into two kinds. Close to the island, the rock and sand off the bays is gentler, and the diving turns slow and observational: marbled rays parked on the bottom, morays wedged into rubble, and, out on the sand at the deeper end of those dives, the endemic rosy-lipped batfish shuffling about on modified fins. Offshore, the seamounts rise out of deep water with nothing around them, and the dive becomes a matter of getting down fast, finding shelter behind a boulder on the plateau and staying put while the blue does the work.

The signature dive is a wait. You settle into the current, keep still and watch the ridge above, and the hammerheads either drop in to be worked over by barberfish and king angelfish or they do not. Nothing about it is guaranteed, which is why itineraries return to the same seamounts day after day until they produce. The other set piece happens after dark, when the whitetips switch from resting to hunting and move over the reef in a pack that follows the torch beams, sometimes straight through the group.

It suits divers who measure a trip in animals rather than water clarity. Visibility swings hard, a thermocline can arrive mid-dive and take both the temperature and the view down with it, and coral cover is thin — the interest is what swims past the rock rather than the rock itself. The schedule is fixed and bad weather is absorbed rather than dodged, since there is no other coastline to run to. Operators generally want an advanced ticket and a logbook with real current dives in it, and the nitrox most boats carry is worth having on the deeper seamounts.