Koh Bida Nai

Entry is usually into Anemone Bay on the east side, a sheltered, shallow lagoon to about 15 metres thick with anemones and anemonefish, the zone boats favour for beginners and skills work. From there the route runs south into a gently sloping coral garden from 3 to 20 metres, threading between large rock formations and a boulder swim-through that operators call the Swim Through Garden. The reef continues southeast into Fantasy Reef, a named outcrop starting around 18 metres and dropping to roughly 28, covered in barrel sponges, whip corals and gorgonian fans, where lionfish, jacks, trevally and barracuda hunt over the drop. The circuit closes on the north and west sides, where the slope steepens into a near-vertical wall to about 25 to 30 metres. Blacktip reef sharks patrol the shallows and outer edges throughout, and zebra sharks, sold regionally as leopard sharks, turn up occasionally resting on the sand.

Koh Bida Nai is the smaller, inner half of the Bida pairing that Phi Phi's own operators and PADI treat as the archipelago's defining dive, and it earns that standing on its own layout. The site reads as one continuous circuit: a training-friendly bay, a swim-through garden, a named outer reef with its own schooling fish, and a genuine wall, all within a single dive. Fantasy Reef is the standout: a named outcrop on the southeast side with an identity of its own, distinct enough that it earns its own name on every operator's map of the site instead of disappearing into a generic reef list.

The swim-through sections between the bay and Fantasy Reef hold a good showing of nudibranchs, worth a slow pace and a macro lens. Fantasy Reef itself rewards wide-angle: barrel sponges and gorgonian fans frame the schooling jacks, trevally and barracuda working the drop-off, and the north wall's soft coral cover holds up well in the available light.

Current strengthens near the island's corners and in the channel toward its twin island; keep pace with your guide and use the sheltered side of the wall when the flow picks up. Scorpionfish sit motionless in the swim-throughs and around the barrel sponges, so keep hands and fins off the rock and sponges as you move through. Advanced Open Water or an equivalent qualification covers the full circuit; Open Water divers can be guided through Anemone Bay and the shallow reef but miss Fantasy Reef and the wall. The dive sits inside a national marine park: entrance runs 200-400 THB for a foreign diver depending on the fee zone, plus a further diving supplement of around 200 THB on top of the boat price, and divers with fewer than 40 logged dives or below Advanced Open Water should expect a camera restriction under Thailand's park rules. Boats routinely pair the dive with Koh Bida Nok and the nearby Hin Bida pinnacle on the same day, and some Koh Lanta operators add a third dive at the Kled Gaeow wreck. The site is sold from Ko Phi Phi Don, Koh Lanta and Phuket, and appears on Phi Phi and Racha liveaboard itineraries.