Our dhow is called Ronaldo. A flag bearing Bob Marley's image flutters from Ronaldo's staff as the monsoon wind gathers and puffs the sail into a bulging beige triangle. Captain Abou adjusts his kikoi wrap, offers a "Bismillah" and a lovely smile and reminds us that these breezes have borne traders on them since the time of King Solomon. Cheetah pelts, porphyry, mangrove poles and people have been traded along these east African routes for 2,000 years, but Abou wants to know: what are we trading now?

We had started out from Diamond Beach on Manda island. Part of the Lamu archipelago, Manda is quiet and understated, unlike Shella opposite, which is a playground for face-lifted millionaires sporting ill-considered sarongs. On Shella they build Arabian Nights fantasy palaces; Diamond Beach is simply a carefully uncomplicated place of nine lovely bandas and a treehouse where, with tact, sensitivity, generosity, kindness and delicious food, Helen and Rachael Feiler help you gently to fit back into your own skin.

Rachael raised the idea of a voyage by dhow the night before last, over a stellar plate of spinach-stuffed calamari, under the huge sparkly Kenyan night sky. A shooting star fizzed and that was an omen - we'd leave at dawn. Ronaldo just about has space for four guests and three crew, and off we slipped into the mirror-calm channel, heading north for Kiwayu island. In Lamu, nearly everything travels by donkey or dhow. Dhows may be ferries with squeeze room for 20 or so laughing islanders. Many are fishing boats which spend this time of year inside the thriving coral reefs. Most have Arabic names, like Ashraf and Naima, for this coast is devoutly Muslim; we also passed a Beyonce, an England and a Lampard, which was more dinghy than dhow.

What are now villages in the Lamu archipelago were once Swahili city-states, each with as many as 20,000 inhabitants. Swahili culture stretched from Mogadishu, on the Horn of Africa, down to Mozambique and beyond, and Lamu got rich on trade. "It is the winds," explains Abou, steering Ronaldo with a bare, sure foot on the tiller: "Between January and April, the monsoon is preceded by the kaskazi coming from the north east. This will blow you from Yemen to Zanzibar and beyond on one sail, with no tacking." After the rainy season, the kusi wind would blow the traders back home.

Dwellers of desert lands transported frankincense, silver and gemstones on the wind, returning with slaves, skins, ivory, rhino horn and precious woods - camphor, teak, mahogany and mangrove for treasure chests and boat building. Abou moors next to the sea wall at Kizingitini to show us where Ronaldo was built. The boatyard consists of two sacking sunshades under which the fundi (master craftsman) directs operations. These have hardly changed since the Queen of Sheba left the land of Punt in a dhow shaped just like Ronaldo. A length of string dipped in dye and flicked to make its mark is the ruler. Holes are bored by a drill worked expertly back and forth with a violin-type bow. Pink-veined mangrove makes the ribs, and teak planks are soaked in shark oil (made by burying shark livers until they decompose) before being bent over a fire.

The scene on the water is unchanged since Solomon's time: dhows lean into the wind, their sails swaying between turquoise water and a sky of infinite blue; above ancient thick green mangroves on low islands village huts are built of woven palm; in the shallows, men with spears hunt flitting silvery fish. But for how long? Abou fields a call on his mobile from Rachael at Diamond Beach who's found a turtle choking on a plastic bag.

At Manda Toto island we pour ourselves off Ronaldo into warm, clear, clean water. There is nobody and nothing about until we snorkel onto the reef, where marine life - zebra fish, flecked lobster, speckled crabs, fish the colour of canaries and fish with plumped-up lips like the ladies at Shella - all busily ignore us.

Abou's boys have cooked lunch on the beach using a charcoal jiko brazier to grill the snapper he'd caught while sailing, and no snapper can ever have tasted so sweetly, moistly epic. The aim is to make Kiwayu by nightfall, and Abou raises Ronaldo's lateen sail to tack upwind while we space out to the sleepy sounds of Mombasa tarab music on the cassette player.

Kiwayu is home to 500 islanders, one of whom, Uncle Kasim, has built a tall rickety treehouse in a magnificent baobab. A twilight swim is essential. The waters of Kiwayu, like the rest of the archipelago, harbour microscopic plankton that create greenish phosphorescence and diamond starbursts around you - a magical marine version of fireflies. The treehouse provides an extraordinary night's rest: water sucks at mangrove roots directly beneath you, the kaskazi pulls and pushes at the worn palm-woven blinds, branches creak, and unseen but fully sensed forms of walking and crawling animal life remind you that this is their tree. You wake to spell-binding views of the sleepy archipelago.

Kiwayu islanders see few mainlanders, let alone Europeans, but greet us the next morning with the pacific dignity that only comes from living on a lush desert island paradise in the Indian Ocean; none is more serene than Shahare Bwana, the schoolmaster, who offers salamas all round and takes us over the dunes to a five-mile-long beach, which is empty save for millions of tall plump coral pink crabs chasing in and out of the waves. Shahare also leads us to the thatched workshop where Kiwayu's women make modest money by collecting washed-up flip-flops to make into bracelets, beaded curtains and mobiles.

We voyage back to Diamond Beach at a clip, the wind in Ronaldo's sail, with Answar the bosun leaning far out on a fixed plank to balance us. Abou talks of visitors, friendly and otherwise: of the Prophet's descendants in the ninth century who used Lamu to disseminate Islam throughout east Africa; of Vasco da Gama in 1498, the Portuguese viceroys that followed him and their attempts to subjugate the islanders; of Yemenis, Omanis, Somalis, Zanzibaris ... all these people left was their genes, which we see in Lamu's handsome faces.

But it is the Qataris that exercise Abou. He calls it neo-colonialism, and you might too. In a wrong-headed scheme Kenya's president, Mwai Kibaki, is proposing to cede Qatar a huge tract of prime land just down the coast on a 99-year lease, where the Qatari's will grow enough fresh fruits and vegetables to feed their nation. In return, the grateful Arabs will build Kenya a deep sea port, the largest and busiest in east Africa. Oh, yes, and the port will be right here in the Lamu archipelago.

"Is this a sin?" asks Abou, already knowing the answer. He points to the fragile coral reefs, the silvery-clean, life-rich water, the thousands of dhows using the wind to move them. "This place was built on trade. But now we're trading paradise for oil money."

He's right, of course, and the time to vist Lamu is right now.

Five places to stay on the beach

Cheap as chips hostel

Stay in Kenya for less than the price of a large cappuccino in London? The Beach Bar Restaurant & Camp Lodge offers ludicrously cheap accommodation, drinks and meals. It is a friendly, family-owned place next to a fishing village. Fresh seafood, prepared on a firewood grill, is available daily or, if you've caught your own (locals will take you out) you can hand it to the chef to BBQ for you. Bandas come with mossie nets and a paraffin lamp.

From £2.82 per night. Available through hostelworld.com whose users have given it an overall rating of 80%.

Resort hotel

Pinewood Village's 36 rooms are set among tropical gardens metres from Galu beach. The hotel is 7km from the famous Diani Beach, the developed hub of tourist activity, but far enough away offer peace and quiet. Nearby Wasini, Kisite, Chale, Funzi and Shimoni beaches are good for snorkelling, scuba diving, deep sea fishing and sightseeing.

Tribes Travel (01728 685 971) has six nights' half board from £910pp including flights and transfers.

Beach house

Watamu Treetops is located on its own stretch of white sand beach, with views of the forest on one side and the ocean on the other. The balcony houses the ensuite bathroom, including an outside shower and

toilet with forest views, as well as a sun deck. The other two bedrooms are accessed by a small bridge and separate spiral staircase.

From £50pp pn through To Escape To (020-7060 6747).

Treehouse

South of Mombasa, 10 minutes beyond the bustle of Diani Beach, The Cove Retreat's 3m-high treehouses are perched above the sand and set against large baobab trees. But despite the rudimentary raw materials, the treehouses aren't basic - each comes with a giant four-poster bed and a whirlpool bath on an open deck overlooking the ocean. Just offshore, several reef-fishing and diving sites can be easily accessed from the hotel, which also has villas tucked into the forest beyond the beach (if the treehouses are full).

Hayes & Jarvis (0871 200 4422) has seven nights' all-inclusive from £1,299pp including flights and transfers.

Luxury camp

Throw back your canvas "door" and step onto the sand . . . the 10 smart tents at Gazi Retreat are right on the empty beach at Gazi Bay. Activities include snorkelling, hammocking and trips into the mangroves. Adults only.

In July, Africa Sky (0870 904 0925) has seven nights' all-inclusive from £1,079pp (or 14 from £1,459) including flights and transfers.

Way to go

Getting there

Kenya Airways (020-8283 1818) flies Heathrow-Nairobi from £349.60 rtn inc tax. Air Kenya (+20 60 6539) flies Nairobi-Lamu from $355 rtn inc tax.

Where to stay

Diamond Beach on Manda island (01273 600 030), beach huts costs from £35pp pn B&B with no single supplement; full board inc drinks costs about £65pp pn. The treehouse sleeps 2-6 and costs £150pn.

Dhow trips

Captain Abou is bookable only through Diamond Beach. Itineraries are bespoke and cost from £40pp per day, inc all meals and drinks.

Further information

Kenya Tourist Board: 020-7367 0931, magicalkenya.com.
Country code: 00254.
Flight time: London-Nairobi, 8hrs; Nairobi-Lamu, 2hrs.
£1= 104.67 Kenyan shillings.