When I arrived in Zanzibar, they told me the apparition I'd seen in the middle of the sea was Chumbe Island Coral Park, an eco-reserve and education centre for local schoolchildren. Established nearly 10 years ago, Chumbe is now recognised as one of the world's most forward-looking and successful eco-tourism projects.

But for a guest, the best thing about Chumbe is the accommodation. A narrow, sandy path leads to your own, private, Tarzan-and-Jane fantasy house. Everything is propped up, lashed together, woven out of grass or made of flotsam and jetsam. Hanging from the ceiling, a rope hammock full of brightly coloured cushions swings in the sea breeze. The upper deck of the banda creaks as you climb the ladder to the bedroom … or rather, bed platform. Looking out towards the sea, the front wall of the bedroom acts as air conditioning and window combined. Lower it with a rope and you can lie in bed and gaze at the sky. This is state-of-the-art eco-architecture with a touch of salvage chic.

Almost all the staff on Chumbe are local Zanzibaris from the fishing community, retrained as marine rangers, who spend two weeks on the island and then one week back in their villages, educating their neighbours about sustainable fishing methods. Catches, which have been steadily declining in Zanzibar over the past few years due to overfishing, are reported to be levelling out.

I barely had a chance to doze off in my hammock before the tide was in and the time was ripe for snorkelling over Chumbe's coral reef, described by Professor Charlie Veron, chief scientist at the Australian Institute of Marine Science, as "one of the most spectacular coral gardens to be found anywhere in the world". I floated for hours, entranced, above a science-fiction world of green and blue forests, valleys of puce, spiky trees and plains of yellow feathery grass. Outlandishly coloured and patterned fish pottered busily around, nibbling fussily at the coral garden or grazing in herds, like cows.

Later, after a dinner lit by flickering candles in jars and accompanied by the sound of the sea, Khamis took me out looking for coconut crabs. These are the world's biggest land-dwelling crustacea, endangered in much of the world, shy and nocturnal. They weigh up to four kilos and feed on coconuts that have fallen from trees, cracking them open with enormous claws. If coconuts get a bit scarce on the ground, they are said to climb the palm trees and snap off the fruit at the top.

After an hour of fruitless searching, I was about to give up when the beam from my torch landed on a claw of monstrous proportions, waving gently at me from inside a clump of undergrowth. I stopped in my tracks and the crab crawled slowly into view, one great leg after another moving deliberately across the sandy path inches from my feet. Its body was deep red, patterned with black wavy lines and more like a lobster in shape than the conventional sideways crab. It was beautiful and at the same time rather horrifying. I was contemplating flight when a ranger appeared out of the darkness and unhesitatingly scooped it up to show me the white abdomen glistening in the torchlight and claws waving in impotent fury. Placed at the bottom of a palm tree, it had an affronted air as the huge legs wrapped themselves around the trunk and started to inch upwards. We left it in peace after 15 metres, still moving inexorably onwards in search of dinner.

Finally, under my mosquito canopy, surrounded by squeakings and rustlings from the jungle around the bed, I lowered my personal portcullis and lay gazing at the stars, caught in the ragged tops of the palm trees as they danced in the night wind to the soft tune of the sea. All I needed now was Tarzan ...

Getting there
Chumbe is located eight kilometres southwest of Zanzibar Town, a journey of around 40 minutes by boat. Zanzibar Island is a two-hour ferry journey from the Tanzanian mainland and fast ferries make the trip four or five times a day in both directions. Qantas flies from Australia to Johannesburg, connecting to South African Airways flights to Dar es Salaam or you can fly on Emirates via Dubai to Dar es Salaam.

Further information: www.chumbeisland.com