Ang Thong National Marine Park

The boat left Bophut pier at eight in the morning, and for the first hour I was skeptical. The Gulf of Thailand in February was flat and blue and the island receded behind us in a pleasant way, but I’d been on enough day-trip boats in Thailand to know the drill — the packed lunch in a styrofoam container, the guide with the microphone who announces everything three times, the stop at the tourist trinket shop. Then the limestone towers appeared on the horizon.

By the time we were threading between the first islands, I had stopped looking skeptical about anything. The karst formations here rise out of the water in vertical columns of grey-green rock, their tops draped in jungle so dense it looks artificial, like a film set where someone got carried away with the plastic ferns. The water between the islands is the specific shade of green that happens when very clear tropical water sits over a shallow white-sand bed, and in the sheltered channels between cliffs the sound drops to almost nothing except the cry of a kingfisher somewhere above and the creak of the boat in its own wake.

The park’s centrepiece, from a purely photographic standpoint, is the saltwater lake hidden inside the main island — Thale Nai, the Inland Sea. You climb a trail through the jungle to a ridge and the lake appears below you, perfectly enclosed by karst walls on every side, its water a shade of green so saturated it looks like someone has dissolved emeralds in it. There is a legend that this lake was the inspiration for the lagoon in Alex Garland’s novel The Beach, which may or may not be true but is at minimum a plausible story. I sat on the ridge for probably half an hour doing nothing except looking at it, which felt appropriately indulgent.

The snorkeling around the park is genuinely good — the park’s protected status means the coral has not been destroyed in the way that so much of Thailand’s reef has been — and kayaking through the sea caves that honeycomb the base of some of the larger islands is something I would recommend even to people who do not think they are interested in kayaking. The caves are navigable at low tide, the limestone walls close enough to touch on both sides, the water glowing green from the light refracted through the entrance.

I went on a standard day tour from Koh Samui and it was perfectly good, but talking to a couple who’d taken a liveaboard for two nights made me regret not staying longer. The park transforms completely once the day-trippers leave — the silence at anchor in one of the sheltered bays, the phosphorescence in the water at night, the possibility of having an island’s beach entirely to yourself at dawn. That is a different trip entirely and one I intend to make next time.

When to go: The park is only open from November to May, when the sea is calm enough for safe navigation. The ideal window is January through March — the northeast monsoon has passed its peak, visibility underwater is at its best, and the midday light on the limestone is extraordinary. Avoid visiting on weekends in high season if you can; the park sees a large volume of day-trippers and some of the popular spots feel crowded between eleven and two.