At 4am, beneath blazing equatorial stars, P&O's Arcadia arrived off the Amazon light vessel. I had gone on deck hoping to hear the pororoca - a collision between river and ocean triggered by the full moon and likened to the crash of siege guns - but our skipper, mindful of his 1,460 sleeping passengers, steered clear. Moving dead slow we made our 15-mile transit of the bar with only a metre between sea bed and keel.

Every eight hours a million tons of silt are dumped on the bar, every 50 days the Amazon discharges enough to form an adobe Everest as wide as it is high. The bar, therefore, remains in a state of constant agitation. I waited for the bump and shudder that would signify we had touched bottom, yet the largest liner ever to enter the world's greatest river finally heaved her 63,534-ton bulk across and picked up speed.

The mouth is 170 miles wide and, though a procession of big trees - mahogany, araucaria, various exotic hardwoods - came wallowing past, toppled by erosion, it was mid-morning before we saw smudges of land. Meanwhile the ship settled into its routine: a clinic taken by the onboard chiropodist, golf chipping in the nets, choc-ices being sold on the sun deck. Then came a bridge announcement: 'At 3pm, after boarding our pilots, we shall cross the Equator.' A woman pointed to an opaline butterfly with the wingspan of a swallow. 'We really are in the tropics!' I noted we also now seemed to be in a river - though the banks remained so far apart they seemed like the shores of different continents.

Over an excellent Goan fish curry lunch I tried to imagine the incredulity felt by Francisco de Orellana, a one-eyed Portuguese who, on a Sunday morning in 1542, went searching for cinnamon in a small wooden boat. Emerging from a tributary 2,000 miles upstream, he and his men encountered a phenomenon which 'came on with such fury and with so great an onrush that it was enough to fill one with the greatest fear'. They emerged from the mouth seven months later telling tales of female warriors even more barbarous than the mastectomised man-killers of Asia Minor. The Amazon was thus named after a Greek myth compounded by Orellana's own feverish imagination.

I recalled some pub-quiz facts. The Amazon and its 1,000 tributaries carry a fifth of all the world's fresh water - 10 carry more than the Mississippi - produce one-fifth of its oxygen, sustain 30 per cent of its forests and one-tenth of its plant and animal species (though each category, bar the first, is diminishing by the hour).

At Santana, a large, leafy town with a scattering of high-rise buildings, we anchored a full mile offshore to collect our officials. A purser spoke of the rapacious Rebelli brothers. 'They were the first Amazon pilots to realise the kind of money they could make from tourism. "Pay up or this ship doesn't move!" Even today the skipper has to hand over $55,000 in cash. And now the authorities want 60 per cent of the takings of our shops - and a big cut of our bar profits. If it goes on like this we may stop coming altogether.'

Only a handful of liners visit the Amazon anyway. It's so far off the beaten track that P&O, selling around 100 cruises annually, does this only once. A circular from the Tours Office said: 'As one of the last frontiers in the world, life in Amazonia is very different to life as we know it in Europe.' It warned of bad roads, poor transport, inadequate guides and 'varying degrees of poverty'.

Yet ships have travelled it for 150 years. Clipper-bowed and brig-rigged, built from live oak, locust wood and hackamack (double-planked and strapped with iron) they later gilded their paddle-boxes and added social refinements: string quartets, champagne bars, and chandeliers that twinkled along the Amazon.

It descends in a series of sweeping parabolic curves, the force of water around the outer rims scouring channels so deep we swept along at oceanic speeds ('17 knots over the ground,' reported a gratified officer of the watch). At times one bank remained a whisp on the skyline as Arcadia, hugging the other, put us within yards of macaws and troupials, blizzards of white egrets, orchids half-smothered by strangler vines, blossom-decked trees taller than Nelson's Column, tiny whey-faced monkeys scampering through a banyan's canopy. Someone even heard a whippoorwill's call.

Leafage thick as thatch neutralised sunlight, made the forest's interior dim and smoky. At dusk a few clearings appeared - each containing a shack where rubber tappers or distillers of rosewood sat by their supper fires. Peering with voyeuristic intensity at them I wondered what they made of us - shining white, 14 decks high, humming with privilege - and was tickled to note most barely even glanced up; we were of less interest than a passing canoe.

Santarém lies on a tributary 15 miles wide and shallow as a pond. Though it possesses a university and a pretty little eighteenth century cathedral it is, in reality, just a toe-hold in a wilderness that reaches all the way to the Mato Grosso. Empty streets and a sense of isolation characterised Santarém on a Sunday morning. But, magically, it soon repopulated itself. Whores in hot pants turned up at the dock, transport materialised. A stout Geordie woman hailed a taxi with a cry of: 'Take me to the jungle, pet!' The rest soon scrambled into coaches and followed. Its proximity had helped sell every berth on Arcadia six months earlier.

Joe distributed walking sticks. Young, black, intense, he warned: 'Once inside always look before you touch; there are flowers that can poison you, and trees with nee dles that will send you to hospital. Last night we had heavy rain; it is very slippery.'

The dozen trudging after him into Amazonia's Green Hell included a used-car dealer, a retired judge and a couple who ran a Bristol off-licence. This dim, airless world with its unexpected sound-effects - exploding rubber pods scattering their seeds - proved to be a giant life-support system. In proper virgin bush, just two acres may contain 3,000 botanical species yet, even here along well-trodden tourist tracks, we saw scores of edible fruits, nuts and leaves, pods which made cashew-flavoured beer, bushes yielding nourishing worms, trees providing sweet water. By a brackish creek we learnt that eight times as many types of fish have been caught in a tiny Amazonian lakelet than in all the rivers of Europe.

Building materials lay to hand: palms like the accashy made good thatch, the caranda strong walls. Swatting at stingless, sweat-drinking bees we noted plants used against malaria and depression, learnt that the forest Indians had identified at least 15 female oral contraceptives, that shrubs containing revolutionary anti-Aids or cancer compounds might be growing only yards away. 'Or maybe not,' said Joe. 'Maybe the last such plants were burnt yesterday.'

In 1996 developers burnt a region the size of Belgium - now the Amazon smoke cloud has reached Antarctica. Since Chico Mendez was shot dead for creating his protected reservas extrativistas, hundreds of other protesters have also been murdered. 'In Manaus you will see the timber factories. And if, one day, you hear they have been fire-bombed, remember - it was me.' Gilberto Mestrinho, a governor of Amazonas state, believed in 'a chain saw for every family'. Responsible for flattening a million square miles of rainforest during three terms in office, he declared: 'I like trees, but they are not indispensable.'

One afternoon, anxious to get the measure of the river, I visited the bridge to meet Luís, our scholarly pilot. He said: 'It changes quickly. One day you pass an island that has been there for 10 years, next day it has gone. So you must keep your hand in; if I am away one month I lose my confidence. Often I still use the old methods - lining up two trees, following ripples and drifting vegetation; these mean depth. This way I could take Arcadia all the way to Iquitos in Peru, 2,200 miles from the sea; a smaller ship I could get quite close to the Andes; the source is there, in a glacier.' An officer muttered: 'I'm doing a fix every three minutes but, according to the charts, we're running over dry land.' Luís winked at me. 'You see? It is playing with us!'

Moments later the air grew dark. Every oxygen atom in an oxygen-rich atmosphere went critical and spontaneously liquefied, heavy water thundered down and made the water foam. We moved in a dense black wall, dead slow, siren blaring. Luís said: 'Our problem is the country boats, they always go fast. The crews are so ignorant! There is no money for education in Amazonas.' The reappearance of the sun provoked a surprise announcement on the bridge PA. 'Ladies and gentlemen, if you look for'ard you'll see we're about to sail smack through the middle of a rainbow.' Luís grinned. 'This river! She can be a real old drama queen.'

Boca do Valerio is an Indian hamlet of a dozen houses - on stilts to offset flooding - overlooking a marsh. As Arcadia's invasion force trampled its territory and walked through its homes, some of the 80 inhabitants posed in cloaks of iridescent feathers for a dollar a shot. Others peddled crude souvenirs; an Essex man looking for shrunken heads (miniaturised, he said, by removing the skull then packing the skin with hot sand) got a few wry smiles from the Indians. P&O's printed Port Guide said we might haggle ('Bartering is possible') and I watched an Old Etonian banker doggedly knock a dollar off a wooden bird carved by an illiterate slash-and-burn peasant. In the end the peasants always gave in. Indeed, part of the charm of the transactions lay in the fatalism of the traders; ultimately you could almost name your price.

Back aboard I wondered what a tiny riverine community would do with the small fortune it had acquired that morning. A steward pointed to a satellite dish half-hidden in the trees. 'Last time I was here they were watching Leeds versus Man United.' He added: 'But it's causing tensions ashore, the chief has even asked us to stop other communities getting in on the act. And there are, uh, cultural implications. Have you seen what's going on aft?' Tiny mahogany canoes, carved with adzes, clustered beneath the stern. Women, some suckling babies, gazed up as female crew tossed down lipsticks, mascara, eye shadow, blusher and nail varnish. 'It's a sisters thing,' a uniformed girl told me crisply.

Some day the Indians might even get José Carreras to sing for them. The one million he got for an evening at Manaus's neo-classical opera house was probably partly compensation for having to perform in a grubby, chronically disorganised city with a chemical edge to its ozone and some of the worst slums in Brazil. Arcadia, inbound, passed the tributarial line along which the inky Rio Negro meets - but does not mix - with the mustard-coloured Rio Solimoes. Pink dolphins may be seen here, even manatees (which Columbus mistook for mermaids). Beyond the floating ice factory lay the giant floating dock, built by a Scot in 1902 and able to adjust, seasonally, to 46ft of fluctuating levels.

Yet Manaus offered access to the igarapes , an intimate neighbourhood of creeks, bayous and half-submerged woodlands. Here, over a drink at a waterside hotel, a retired teacher told me about Amazonia's problems - the huge areas to be flooded for hydro-electric schemes, the human suffering caused by the wildcat garimpeiros who used mercury to prospect for riverine gold, the heavy-lift Russian helicopters soon to start plucking trees from areas previously protected by their remoteness. Only half joking, he said: 'In 100 years, much of our rainforest may be turned into golf courses.'

As Arcadia headed back to the Caribbean, many of her passengers - golfers included - had been turned into ardent rainforest activists.

What it costs

Alexander Frater was a guest of P&O Cruises on board Arcadia on the 15-night Amazon and Caribbean cruise. A similar 15-night cruise - the Amazon Adventure - runs from 19 January to 3 February. The fly-cruise departs from Barbados on 19 February with ports of call including Trinidad, Santarem, Alter do Chao, Manaus, Boca do Valerio, Ile Royale and Grenada. Prices for the cruise start from £2,175 per person and include return flights and all meals on board. Other Caribbean cruises are available on Arcadia throughout the winter.

For a P&O Cruises brochure, call 0990 726726. To make a booking call P&O on 020 7800 2222 or visit an Abta travel agent.