Koh Rong & Koh Tang

There are two quite different trips on offer. Inshore, day boats run from Sihanoukville or off the beaches on Koh Rong and Koh Rong Samloem for two shallow dives, often sharing the boat with a course group; the hops are short and the operation small, a handful of divers rather than a fleet. Going offshore means committing to an overnight trip that leaves in the evening or before first light, sleeps aboard and fits several dives into a long weekend.

Inshore the seabed is boulder, rubble and silt on gentle slopes, and the water carries sediment off the mainland, so you dive it looking down and close in rather than out into the blue. Offshore the structure improves: hard coral over boulder reef, shallow fringing sections around the islands and better-defined slopes to work along. Koh Prins is dived for wreckage lying on the seabed, broken up rather than any intact hull, and accounts of what the metal actually is do not agree.

It suits divers happy to trade polish for quiet: offshore sites can go days without a boat on them, guides know the rubble intimately, and nobody is queueing behind you. Tolerance for a long ride on a basic boat matters more than dive skill. Trips also depend on numbers, since offshore departures usually go only when enough divers sign up, so anyone travelling in the shoulder months should contact centres in advance rather than turn up assuming a boat will be going.