Chinese and Vietnamese pirates were holed up in the archipelago - not surprisingly: it's hard to imagine a better environment for playing seaborne hide-and-seek. Cruising the islands, you constantly see boats appear from and disappear behind the towering rocks.

While flushing the buccaneers out of this marine maze, the French were dazzled by what they saw. Already in the local shipping business, the Roque brothers from Bordeaux built four flat-bottomed paddle-steamers to transport cargo and provide pleasure cruises in the gulf.

Today, only one boat deliberately harks back to that colonial era: the Emeraude, a close replica of a vessel of the same name from the halcyon days of French Indochina. Stepping aboard, you are instantly in a world of polished board decks, gleaming brass fittings and pressed white duck coats. Brass fans whir, the signs say 'pont principal' and 'pont superieur' - instead of 'main deck' and 'upper deck' - and the captain gives a Gallic welcome over the tannoy: 'Bienvenus a bord.' The boat steams out of port towards a craggy mountain range in the distant sea. Within 30 minutes, we're among the rocky pillars of Ha Long, with all eyes on a towering wedge-shaped island exotically topped by a Chinese pavilion.

Every boat seems to head for a horseshoe-shaped inlet enclosed within a great fortress of an island: the so-called Surprise Cave. The Emeraude - too large to go into the lagoon - anchors outside and sends in its passengers in boxy little tenders, which spend the rest of the time tagging along in the larger boat's wake.

The wise stay on the lounge deck, with exotic cocktails in hand, lolling in rattan chairs. The real luxury of the Emeraude is the space, four decks to roam, including a canopied lounge area with a central bar and a deck-chaired eyrie for sunbathers on top. All this gives unrivalled viewing opportunities as the boat gently eases through the calm waters, passing the lovely weirdness of an ancient flooded mountain range.

Only the captain, with his detailed charts, has any idea where we are and where we're going; for us dreamers, the archipelago is an enigma. Some sources reckon there are about 3,000 islands while Unesco, which declared Ha Long Bay a World Heritage site in 1994, gives a figure of 1,600.

As to their origins, the geologists are, as ever, trounced by folklore, which recounts the gods sent down a family of celestial dragons to defend the land from invaders. The dragons spat out jewels and jade into the sea, which turned into a multitude of jagged islands and islets that repulsed the enemy. Legend has it a strange dragon-like creature called the Tarasque still lurks in the bay, and local fishermen swear they've seen it.

Dawn reveals a great massif rising precipitously out of a grey sea. As we edge out into open water, visions of distant misty tall rock formations parade before our eyes, drifting and morphing with our progress.

The sun rises and turns all from grey to gold. A lone figure, in a conical hat, rows a basket boat across a mirror of glistening waters.

The original Emeraude sank in 1937, but you don't feel the slightest hint of danger in this elegant tub. Indeed, this round-ended, layer-cake boat is so stable you never find yourself staggering drunkenly about. Unless you are drunk, of course, which can easily happen given there are two bars for fewer than 80 voyagers.

Stormy weather would presumably change this, but this trip is decidedly plain sailing ... apart from the delicious decadence, nature at its most magnificent, the tropical languor and the transcendental experience.