Koh Bida Nok sits within a marine park and has some of the best dive sites in Phuket. This dive site is just south of Phi Phi Ley and although it means outer island in Thai, it is actually just a rocky outcrop. There are two main dive sites on the island with a sloping reef found on the south eastern side going down to about 25 meters, and a wall going down to about 30 meters in the northwest. The current is normally quite calm and does not get very strong and this is one of the most popular dive spots amongst the Phi Phi islands.
The northern wall reef provides great wall diving with abundant corals along the wall. Sea snakes, lionfish, jackfish, nudibranchs, and sweetlips are found along the wall. There is a small cave along the wall, which divers can enter and explore but they should only do so with proper training. Along the reef of the south eastern side, the reef turns into coral bommies and amongst these divers can spot zebra sharks and stingrays in the sand.
In the shallower parts of the dive sites, it is common to spot leopard sharks sleeping on the sandy bottom and turtles swimming around the coral. On both sites, divers can spot snappers and fusiliers in large groups and if divers explore the cracks and crevices they can easily spot Ghost pipefish and scorpionfish.