Koh Mai Si

Ko Mai Si is an uninhabited island off the northwest coast of Koh Kood in Trat province — Gulf of Thailand waters, not Andaman, and a boat-trip destination rather than anywhere you can stay overnight. Local dive operators run it as a stop on half-day snorkelling trips out of Koh Kood.

The island’s underwater landscape leans toward boulders rather than coral gardens — dramatic rock formations that dive operators describe as home to stonefish, eels, nudibranchs, and stingrays. That’s a different draw than the classic reef-and-tropical-fish scene: less colourful coral, more interesting marine life tucked among the rocks. Watch out: stonefish have venomous spines and are easy to miss against the rock — water shoes are worth the hassle here, and avoid putting bare feet down on the seabed.

Trips typically combine Ko Mai Si with Ko Mai Si Lek nearby and sometimes extend to North Beach (Klongtin Beach) on the same run, so you’re rarely visiting this one island in isolation — it’s part of a west-coast snorkelling circuit rather than a single-stop trip. Expect no shops, no shade structures, and no facilities of any kind; whatever you need for the day comes with you.

Insider Tip: Book through a dive shop rather than trying to arrange an independent longtail — the boulder terrain here is easier to snorkel safely with someone who knows where the rocks and any current are.

Ko Mai Si sits within Koh Kood’s island group. Nearby Koh Kradat and Muko Rang are common add-ons on the same island-hopping trips if you want to turn this into a full day on the water.