Koh Thalu

Koh Thalu is a small limestone island inside Mu Ko Chumphon National Park, about 13 km off the river mouth of Chumphon. Its name — “thalu,” pierced or through — comes from a natural rock tunnel cut through the island’s cliffs that boats can pass beneath and swimmers can swim through on calm days. It’s a day-trip-only dive and snorkel site, with a national park gate fee (200 THB for foreign adults, 40 THB for Thai adults) rather than free entry.

Watch out: don’t confuse this with the better-known Koh Thalu off Bang Saphan Noi in Prachuap Khiri Khan, which has an overnight resort and a different fee structure entirely. This Chumphon island, inside the national park’s southern waters, has no buildings, no jetty infrastructure, and no way to stay the night — everything about a visit here happens between a boat’s arrival and departure.

Coral gardens ring the shoreline close enough to reach by snorkel fin from the boat, with staghorn coral and anemones that host clownfish in the shallows. Divers occasionally spot blacktip reef sharks passing through deeper water off the drop-offs, and fusiliers and angelfish move in loose schools around the reef edges. Between March and April, seagulls nest on the exposed rock ledges — worth timing a visit around if you want to see the colony active.

Because there’s nothing built on the island, every trip runs through a tour operator or private charter from the Chumphon river mouth, roughly a 30-45 minute run by speedboat depending on the vessel and sea conditions. Most day packages bundle several islands in the archipelago together with lunch, snorkel gear, and the park fee already included, which is the easiest way to visit — going independently means arranging your own boat and paying the gate fee separately at the park checkpoint.

Insider Tip: aim for February-April or October-November. Outside those windows, Gulf of Thailand swell can make the rock tunnel too rough to swim through safely, even when the boat trip itself still runs.

Bring reef-safe sunscreen, a rash guard, and drinking water — there are no shops or shade structures on the island itself, just rock, coral, and the tunnel that gives Koh Thalu its name.