Phi Phi & the King Cruiser

Almost everyone dives this stretch as a day trip. Boats tend to leave Phuket's southern piers early and run the open-water rocks first, before the wind gets up and before the day's traffic has stirred the sand, then work back toward the limestone for the later dives. The order is not fixed: tide, swell and how many other boats have booked the same moorings all get weighed on the morning, so the plan you were sold can change at the pier. Shops based on Phi Phi Don skip the crossing and start on the walls instead.

The two halves of the area dive nothing alike. The submerged rocks stand off a sand plain in open water, so they take the full tide, carry soft coral and fans on their exposed faces, and often sit under greener, more plankton-heavy water. The islands are drowned karst, the grey rock carrying on below the waterline much as it does above, split by clefts and undercuts, with cleaner water and fish stacked tight against the stone. Booking both in one day is the point; they ask different things of you.

The ferry is what most people book for, and she has changed. Decades in warm water have thickened every railing with growth and brought down sections of the upper structure, so guides generally work the outside of the hull and use the open, collapsed sections rather than thread anything enclosed. Visibility over her is rarely a given, since this is open water off a sand plain with no shelter. On a poor day the hull appears out of the grey a few metres away, which is either the best moment of the trip or the reason you would rather have stayed on the walls.

It suits divers who want range in a short stay rather than one reef done properly: steel, karst wall, sand and coral pinnacle inside two or three days out of the same hotel, without committing to a week on a boat. It is not quiet diving. Several boats share the same moorings on any workable day, and the shallower reef shows it. Photographers usually do better on the island walls, where the small stuff sits on a vertical face at easy depth and nobody is finning over the frame.