Turning my head, I noticed that our lure, suspended from a marker buoy, was glittering seductively just above my shoulder. I moved at what I like to think was a dignified but rapid rate to put some distance between it and me. Should a hammerhead or whitetip come belting along expecting a tasty snack, I was not anxious to be swept away by its enthusiasm. I wanted to enjoy the view.
Banc Vailheu is reckoned to be one of the world's great wall dives, some say the finest in the Indian Ocean. It is a sea mountain, blasted by volcanic forces more than a mile up from the ocean floor, and stretching underwater for nine miles. Banc Vailheu can reach within 7m of the surface at low tide. One day it may become the fifth island of the Comores archipelago. The mountaintop is a mile across, and drifting over it when the current is running is said to be exhilarating. And it is exciting hanging off the wall at around 30m, trying to take in the activity among the virgin coral while at the same time scanning the open sea for sharks, rays, dolphins and, if you are a born optimist, whale sharks.
But despite the shiny lure and excellent visibility - perhaps because of that viz, as our guide Noel suggested - no such creatures swam into view that day, though we did see an interesting selection of fishermen's friends - kingfish, tuna and barracuda - on patrol.
The spinner dolphins that had followed the boat out were not to be seen; instead there were unicornfish, clouds of snappers, fusiliers and the odd trunkfish. I stopped a while to watch a big tail-barred parrotfish battering away at the coral with its fins like a marine Mike Tyson.
We spent an hour and a half on the wall that day but, spectacular as the view was, I am not convinced that either of our dives at Banc Vailheu was on its happiest hunting grounds. Lacking GPS on our 12m dive boat, both sites had been found by a combination of fixing on the presidential palace on Grand Comore - splendid enough to be visible even 12 miles away - and canvassing the views of fishermen who seemed to have ventured recklessly far from land in their small craft. It was good, but I suspect it gets better.
When I first mentioned to people that I was off to the Comores, few knew where it was but all had theories on its pronunciation (Comores is French and used there, Comoros is English).
And after a week and 13 dives there, I reflected on the plane back to London on the difficulties such global backwaters face in capturing the attention of the wider world.
So I was shocked on arriving home to find that the islands had leapt into international focus overnight. There were the people with whom I had been diving, talking heads on the Nine O'Clock News.
At 3pm the previous day all three of Island Ventures' dive boats had been out, the divers down on their afternoon forays. One can hardly imagine the reactions of those on Treasure Cove, the 15m-deep bommie nearest to the centre, when they heard a bang and surfaced to find half a Boeing 767 sticking out of the water, metres away.
When they had told me at the dive centre that they could do with another wreck in the area, this was not what they had meant. Divers were prominent among the rescuers of 50 people from the wreckage of the hijacked Ethiopian airliner. But 75 died in the crash, the worst death-toll ever to result from a hijacking.
The tragic human cost naturally shook those involved. Little damage was caused to the reef, however, and it was very much business as usual when I called a few weeks later to find that most of the wreckage had been cleared.
"All you can see now are some business-class seats sitting on plate coral," said Guy Fotherby grimly. Guy manages the Island Ventures Club Nautique diving and watersports centre on Grand Comore with his wife Caroline. And a tightly run but friendly operation it is.
Island Ventures has something of a monopoly on the diving in this part of the world. Until Banc Vailheu pops out of the water there are four islands in the Comores group, which lies between Mozambique and Madagascar. Three - the main island Grand Comore, Moheli and Anjouan - form an independent Islamic republic, while Mayotte remains French.
From Grand Comore, Island Ventures can arrange cruises that include diving to all the islands. It has three dive boats: an 8m monohull, an 8m catamaran and the twin-diesel 12m boat.
Of the 12,000 annual visitors to Grand Comore, some 11,000 head directly north to Sun International's luxurious Le Galawa hotel and adjoining Maloudja bungalow resort. Island Ventures forms part of the package.
The hotel sleeps 400 and cuts few corners - it employs 440 staff. It is a virtual state within a state, and refused the government's request to provide the staff with their own welfare system. As the assistant manager put it, "Where else would hotel management be like running a country?" Even Island Ventures, which offers a wide range of watersports besides diving, has more than 40 staff, including the five instructors. Don't expect to carry your own kit.
Up to 70 per cent of visitors are South African; but, as the value of the rand declines, the resort is anxious to attract more affluent Europeans. French and Germans come; it would welcome more Brits. "We believe diving is important to attract people from Europe," says Tourism Services Manager Robert Ambroise.
Is it worth the effort? After all, this is an 18-hour trip, flying over the Red Sea to make your southbound connection after a three-hour stopover in Dubai. Caroline Fotherby's answer is an emphatic yes. "Divers can get away from it all; it's that bit further than the Red Sea but different. The diving is relaxed and civilised."
Caroline is the only person in the world to have been bitten by that rarest of deep-dwellers, the coelacanth - the pickled culprit now glares at visitors to the boathouse from its tank (see feature overleaf). She agrees that the Comores is not for out-and-out thrill-seekers, or "big-game hunters", as she disparagingly terms them: "I get so mad, I just want to shake those people," she says.
But the colour and diversity of the reef life will appeal to marine biologists and macro photographers alike. Reef dives by day, and especially by night, are spiced by excursions to the single wreck, the Masiwa, and the exciting promise of Banc Vailheu.
Come in August and they say you can dive with humpback whales on their way to give birth. But we were there in late November, at the butt-end of the balmy season that begins in May. Rough weather, including cyclones, can occur between December and April, although selective amnesia seems to set in when the subject is broached on the island.
There was no sign of rain until our last day - the days were hot and the nights warm. Two evenings running we visited the Mosque, a coral temple a short boat ride south, to dive as the sun set. Never more than 12m deep, the first night with ten torchbeams converging on each electric ray or nudibranch was a bit frenetic but whetted the appetite. The second night, with fewer divers, was magical.
In the first small outcrop a massive parrotfish was wedged impossibly into a crevice; from the other side an equally large honeycomb moray glared out. From there on, every square metre of the Mosque seemed to yield a parade of creatures from Central Casting, eager to audition - electric rays, a tiger snake, lionfish, starry dragonets, sizeable crayfish, lobsters and hermit crabs, a magnificent Spanish dancer.
The visitor to Grand Comore is reminded at every turn that the place erupted into being. Karthala, the world's biggest active volcano, broods over the island. It can be visited if you are willing to camp near the mile-wide rim overnight. The island's bays of fine white sand are separated by black lava spits, which extend into the sea to form the basis of the reefs.
The results are there for all to see on much-dived but healthy-looking reefs. Black Coral Cave, 45min south of the centre, is one close to the shore that drops and levels off at 18m, where you will find the opening that gives the site its name. The wall drops off to unknown depths, fan corals starting at 35m. An unusual sighting here was of a frogfish.
During his 11-year military rule, the trawler, the Masiwa, was used as a supply boat before being abandoned for a decade on the reef off Moheli. 
