Mi'il Channel

The dive is set by the tide rather than by a departure time. The channel cuts through the barrier reef with a sandy coral rubble bottom and vertical coral walls on either side, and the floor runs between 15 m and 30 m. At slack water the current is barely perceptible and the walls can be worked slowly; the nooks and crannies in them hold lobsters and octopus.

On an incoming tide the same water becomes a drift. You drop in and let the flow carry you down the centre of the channel, close enough to the middle that both sides stay in view at once. Mantas are the reason the boats come, but the outer reaches of the channel are where the rest of the traffic turns up: hawksbill turtles, large barracuda, reef sharks, eagle rays and schools of jacks and snappers.

Timing is the whole dive. The same channel that is barely moving at slack can rip to as much as 4 mph (6 km/h) once the tide changes, so the boat works to the tide table and the window it gives you. Visibility runs from 9 m to more than 30 m. Across the tide cycle the flow moves between 0 and 6 km/h, which is the difference between hanging on the walls and taking a one-way ride down the middle.