Tarutao Marine National Park: A 51-island Thai archipelago that remains untouched by tourists

The anchor rattles, the ship stills and limpid blue seawater laps over the water-sports deck at the back of Panorama II, warm on my ankles as I clamber into the Zodiac. Ahead, the sea is a startling emerald colour, the beach white as talcum powder, the jungle-clad hillsides behind dark green and frothy with palm trees. Rounded black boulders tumble at the edge of the bay, as if placed by a Zen landscape designer.

It is day five of my Peregrine Adventures small-ship cruise through the Thai and Malaysian islands, and we are in Koh Adang. It's as if we've slipped through a portal in time, back to the way Thailand was 30 years ago. Koh Adang has a low-key resort, and a national park service campground and basic bungalows. Somewhere, but not here. Here there is only a single long-tailed boat afloat off the beach, its occupants nowhere to be seen.

We're being cast away for the day, but no one is complaining. Peregrine Adventures commits itself to small-group, responsible travel, local experiences and excursions off the beaten track – or well-sailed wave in the case of its new south-east Asia cruise. Panorama II is a 50-metre yacht and carries just 49 passengers. Now we've left even the yacht behind. The bright-red Zodiac coughs us up on the beach and falls silent. Only the cicadas hum.

You hear a lot these days about the tourists who plague some Thai islands like a locust swarm, crowding them out and damaging the environment. Yet Tarutao Marine National Park has a whole archipelago of 51 humped islands, only one of which caters to mass tourism. It's one of Thailand's least-developed regions, with well-preserved rainforest that is still home to fishing cats, mouse deer, langurs and macaques.

Its reefs are filled with giant clams, turtles and brightly coloured anemones and corals. I borrow Panorama II's snorkelling gear and, just off our beach, find orange boxfish studded with black dots, batfish trailing yellow fins, and fierce little neon-blue fish the size of my finger.

It's hard to know whether Koh Adang is more beautiful below the water or above. See-through crabs scuttle on its shell-scattered beaches, birds twitter in the palm trees, and on the horizon the sheer cliffs of other islands are dark blue against a bright-blue sea. Some of my fellow passengers kayak or paddleboard around little coves where the emerald water is as clear as vodka and so warm it invites you to fall overboard.

We return to our ship satiated by sun and salt and swimming. There isn't another ship in sight, let alone the behemoths many people associate with cruising. This is an anti-cruise. Panorama II has no casino or speciality restaurants or sequined shows. It has passengers barefoot on deck, sociable over drinks or lazily turning the pages of a magazine as the wind ruffles their hair.

In the late afternoon, the anchor is winched up and we set sail for Langkawi, which is just over the border in Malaysia and right back on the tourist trail. You can forget everything you hear about this part of the world being overrun with visitors though. For the moment, we're the only ship on a shimmering silver sea whose edges have been set alight by an orange sunset. Cocktail glasses clink and Panorama II's sails snap in the breeze. Then the sea turns dark and dinner is ready on the deck. The day is gone, but it will long linger in the memory of those lucky enough to have lived it in such a beautiful place.