I dived off the yacht's hull beneath a 200-foot limestone cliff, its face broken by mangroves and cacti, which sprouted implausibly from the sheer rock. Abuzz with the thrill of bouncing across choppy sea at 30 knots, I swam, fins on and face mask down, towards a small cave mouth at the island's water line where the waves changed from emerald to a navy hue and slopped, foaming, over rocky teeth. It was a Boy's Own mission of discovery.
Inside the grotto, a little sunlight flickered over high walls plastered with clam shells. The cave narrowed into an inky tunnel, and our courage faltered. But a torch beam revealed fruit bats in a roof cavity, crowded thick together, asleep.
I swam in the salty, hot waters of Phang Nga Bay eight times that day. Its 42 islands – rock monoliths covered with Jurassic scrub and crab-eating macaques – tower skywards out of the shallows. Underwater, schools of lemon damsel, parrotfish and Andaman sweetlips inspected we bobbing snorkellers, then fled terrorised when confronted by two wide human eyes. A barracuda slunk by baring its teeth; a five-foot water monitor lizard hugged the shore, searching for a place to land. The elusive dugong or "sea cow" can occasionally be seen grazing in these waters, but you'll need patience and a slice of good fortune to spot it.
Later, the three-man crew served a gourmet lunch on deck – white linen, canapés, four courses, a plucky white wine – as dozens of sea hawks and two eagles circled, occasionally plunging into the waves, talons bared. It was startling, so far out, to watch tiny yellow butterflies flutter over the surface.
Phang Nga Bay had one further treat: mineral cathedrals hidden in the heart of each island, beyond dripping stalactite fingers and tunnels navigable only by canoe. These hongs – glittering lagoons with high sheer sides – form where the limestone collapses into the sea: a seemingly ancient world.
Then, just when you think you've fled the bustle of the modern world for five minutes, you paddle around the corner and bump into a teenager selling mangos and warm Coca-Cola off the back of a long-tail skiff, a full 20 miles from the mainland.
Down in Thailand's rural south-west, away from Bangkok's flattening humidity and snaking red brake-lights, life is agreeably slothful. Backpacker hostels and low-budget rooms used to be the means to explore Phang Nga Province's mix of rainforest, temples, sprawling paddy field countryside, beaches and dive sites. That budget market is strong, particularly among young travellers looking for adventure, but recent years have brought the emergence of five-star hotels.
The Sarojin resort, near the town of Khao Lak – 50 minutes from Phuket airport – has 56 suites, half with private pools, set in 10 acres of grounds next to the Andaman Sea and off the beaten track. Its appeal is simple: impeccable hospitality and activities limited purely by one's imagination and budget.
The hotel's ethos, propagated by the owners, British couple Kate and Andrew Kemp, is that guests are friends staying at a private estate (albeit paying for the pleasure). Teenagers are welcomed, but not children.
Lazy types vegetate by the infinity pool, sip fruit elixirs on the six-mile beach or get pulverised by deceptively tiny masseurs in palm-thatch cabins. Inquisitive guests set out by 4x4, mountain bike, scooter or elephant to explore Khao Sok national park's rivers and waterfalls, braving the occasional fierce fong-tok (downpour). Between November and April there's also the option of a trip to the nearby Similan Islands for diving with 60ft-visibility.
But the most popular of the Sarojin's expeditions is a Thai cooking lesson in the forest. Students are propelled at neck-jolting speed along the Takuapa River in spluttering narrow "pleat" boats, past darting egrets and bright green bamboo vipers coiled in the trees.
The passengers are then ejected onto the riverbank next to Takuapa Old Town market, where chef Nattaphong Ritthisuntorn tours trestle tables creaking with pungent produce. He shouts to be heard over the clattering of a rusty fan. (His English is excellent: he spent two years working in, of all places, Berkhamsted in Hertfordshire.)
That heady aroma is coriander, he explains, pausing beside a green mountain of aubergine, kaffir lime leaves, long beans and lemon basil. The show-and-tell of ingredients continues: eels thrash dementedly inside a bucket; fiery bird's eye chillis ("Thai peppers") are piled unassailably by cardamom pods, ginger roots, shrimp paste jars and pyramids of boiled eggs stained blue and pink. Then come 20 extraordinary feet of custard apples, guava, and vibrant pink-and-green dragon fruit. The football-sized durian fruit, a South-east Asian delicacy, is banished from the main market. Its stench has been likened to a combination of rotting carrion, fermented onions and ripe cheese.
The cooks are packed back in their boats to whizz down the river to a clearing in the forest. There, the ingredients are laid out in baskets to chop and stir, a sort of cook-by-numbers for culinary simpletons.
Wearing a ludicrous white chef's hat and cradling a goblet of Chablis, in the manner of the late Keith Floyd, I leant over the propane stove, tongue planted in cheek, and tried not to cock up my starter, a grilled beef salad with lemon chilli dressing called yam nue yang. Having safely deposited the charred remains in half a coconut to cool, it was on to the mains: green chicken curry with aubergine and coconut milk (gaeng kiew warn gai) and then the pinnacle, tom yam goong – hot and sour prawn soup.
Bowls of curry pastes, soy sauce and sesame oil were splattered across aprons; spice-covered pestles were waved in anger... Less lemon! More chilli! Then we were ushered away from the kitchen and towards a crisply-laid dining table by the riverbank. Although I do say so myself, it tasted bloody good.
Of The Sarojin's 245 staff (almost all of them locals), 40 are chefs, allowing the resort to cater for guests' requests for barbecues and intimate tables laid on the beach or at waterfalls, where guides will discreetly prepare dinner.
The Sarojin has two restaurants: a candlelit one on the beachfront with different Asian cooking stations; and the second beneath a gigantic ficus tree, where diners are serenaded by thousands of frogs partying in the vast lotus pond.
For those enfeebled by too much good living, a former Moulin Rouge dancer implausibly named Jowell Philemond-Montout floats about in white linen and leads yogic stretching classes on the beach each morning, among the anxious, scurrying sand crabs. His job title, "The Imagineer", makes him a sort of activities-coordinator-cum-philosopher who conjures up romantic endeavours for guests. Bamboo rafting? Tracking white-handed gibbons in the Lam Ru forest? Meet you in reception at 7.45. Marriage proposal at 5,000 feet in a hot-air balloon? Same time, but bring your Black AmEx card.
The staff specialise in little surprises when you return to your room at night: rose petals floating in a deliciously cold bath, or the menagerie of animals constructed from hand towels left on the bed – crabs, sharks, rabbits.
Khao Lak and The Sarojin were flattened by the 2004 Boxing Day tsunami. Phang Nga province was the worst hit in Thailand: 4,200 people died. The resort had been due to open to its first guests days later. Fortunately staff had been given Christmas off, although many of them lost family members. A solitary item of debris remains: police boat 813, left as a memorial where the 30-foot wall of ocean hurled it a mile inland. Elsewhere, hotels have been rebuilt; the forest grows back; tourism has recovered, and serenity has returned.
The joy in retreating so far from the bedlam of Bangkok and high-rise Phuket lies not in the transient luxuries – however pleasant – but in glimpsing a calmer Thailand.
Hire a scooter and set out along the quieter countryside highways, past flatbeds packed with grinning workers, sheets of white rubber drying on racks and monks walking the hard shoulder. Visit a cashew-nut factory, where the women won't look up from their delicate task of cracking shells.
At Khao Lak's night market, tuck into snacks wrapped in banana leaves: you need neither a command of the language nor a menu in English. Simply make your choice from stalls serving sizzling, steaming and smouldering dishes. Just watch you don't opt for the succulent maeng-da – insects – or those Thai smiles will widen.
Travel essentials: Thailand
*The writer travelled with Kuoni (01306 747008; kuoni.co.uk ), which offers 10 nights in Thailand from £1,597 per person. The price includes return economy Thai Airways flights from Heathrow to Bangkok, transfers, three nights' room only at the Shangri-La Bangkok and seven nights' room only at The Sarojin Khao Lak.
*Bangkok is served by Thai Airways (0870 606 0911; thaiairways.co.uk ), British Airways (0844 493 0787; ba.com), Eva Air (020-7380 8300; evaair.com ) and Qantas (0845 774 7767; qantas.com ), all from Heathrow. Regional departures are available with airlines such as Emirates (0844 800 2777; emirates.com ) via Dubai.
*The Sarojin, Khao Lak, Phang Nga, Thailand (00 66 76 427 9004; sarojin.com ).
Cooking lessons at the waterfall cost £240 for two people, or £300 including boat travel, a morning market visit and lunch on the riverbank.
Chartering the 38ft Lady Sarojin launch, inclusive of crew, lunch and an open bar, with a maximum of about 10 guests, costs £800 per day.
*There is plenty of budget accommodation in the region, double rooms cost from around 1,000 baht (£9), room only.
*Exploring Phuket and Phi Phi, by Oliver Hargreaves (Odyssey, 2008, £14.95).
*Tourism Authority of Thailand: 0870 900 2007; tourismthailand.co.uk