Koh Bida Nok

Koh Bida Nok is the larger of Phi Phi's twin Bida Islands, a steep limestone islet paired with the smaller Koh Bida Nai. Most dives start in the sheltered bay on the south side, a shelf running 3-8m that boats favour for beginners and mixed-level groups. From there the dive opens out: cliffs on the southwest side drop straight into the blue, covered in soft coral and anemones in purple, aquamarine, pink and blue, with a vertical swim-through cutting through the rock. A coral ridge divers call the Finger Reef extends south from the bay and keeps dropping past 20m. Leopard sharks turn up resting on the sand or moving along the walls, and blacktip reef sharks patrol the reef edges on both sides of the island, often close enough that a wide-angle lens outperforms a macro one. The north side and the channel toward Koh Bida Nai are more exposed, with stronger current and depth running into the high 20s, better suited to divers who are comfortable working at that range. A narrow cave on the north side and a longer one cut into the west wall sit outside this route entirely: overhead features for cave-trained divers only, not part of the standard dive most visitors do.

Bida Nok and Bida Nai together are the dive most people mean when they talk about Phi Phi, and Bida Nok is the half built for a mixed group: a genuinely easy entry bay on the same island as a wall that runs past 20m and a shark sighting record solid enough to earn its own line on the dive board, separate from a catch-all reef stop. Its twin has a sheltered bay too, so the real difference is what each island is known for beyond that: Bida Nai's circuit builds toward its own named Fantasy Reef stretch and boulder swim-throughs, while Bida Nok adds two overhead caves and the Finger Reef ridge to its bay-to-wall route. Bida Nok gives a boat carrying an Open Water diver and a divemaster candidate somewhere to put both of them on the same trip, then rewards the stronger diver with current, depth and two shark species that turn up on most trips out here.

Book with the site named specifically. Outside high season it is not fixed on every operator's itinerary, and asking for "Bida Nok" (or Bida Nai) by name is what locks in this exact site over a generic Phi Phi reef swap. A wide-angle lens is the better choice for the blacktip sharks along the reef edge, though a 2025 Thailand-wide rule bars an underwater camera below Advanced Open Water or 40 logged dives, so check where you stand before you pack one. The two caves are worth knowing about even if you never enter them: the north-side cave requires Cavern certification and the west-wall cave is long enough that going in without dedicated cave training and an experienced professional alongside you carries real risk. Expect the current to pick up noticeably at the Finger Reef and in the strait separating the two Bida islands, and keep close to your guide through both. Triggerfish nesting nearby can turn territorial; give one displaying at you room instead of pushing past it.