My hand hooked firmly into his weight belt. Tom floated a foot or so above me, legs bent behind him, arms outspread like a skydiver in freefall. His eyes huge behind the mask, he gave the signal for "look at that", pointing towards the medusa head of a purple anemone beneath us. Pouting among the mass of pullulating fronds was a small, familiar orange fish with two white bands across its middle. So that's why he was excited. To hell with global warming - we'd found Nemo.
To a child, diving Australia's Great Barrier Reef is unforgettable for all sorts of reasons, but top of the list must be seeing something that simply may not be there much longer. Coral bleaching due to rising sea temperatures, the depredations of the crown of thorns starfish, pollution from urban water run-off, commercial fishing - all are having a negative effect on this, the only congregation of living organisms so large it can be seen from space. Number two on the list, though, is meeting cartoon film stars in the fishy flesh.
We had gone in search of clown fish - a Nemo's proper designation - with the help of the team at Lady Elliot Island, a minute coral cay at the southern end of the Barrier Reef that looked, as we made our landing approach in a cramped Seair twin-prop, like a splodge of guano dropped in the sea. (It's a suitable image: the island has no more than 50 guests at a time, but houses 100 times that number of birds, who skreek and splat noisily all over their rookeries in the casurina trees.) As the plane banks towards the grassy airstrip it gives you a grand tour of the island - which, being less than a mile wide, doesn't take long. In quick succession, you putter over a turquoise lagoon, a bone-white rubble beach, a lighthouse, trees, birds, thatched bar, cabins and tents. Bingo - you've landed.
Bingo is what Tom and his brother Jack played on the first night - Reef Bingo, hosted by a caller who also happened to be a very enthusiastic marine biologist. "Look at your cards, everyone," she said. "Who's got the single-cell algae that live inside coral polyps?" "Me! Zooxanthellae!" shouted Jack. ("Is that the same as Legs Eleven?" muttered my husband, and left for bed.)
This is how we learned about the reef's chief enemy: the overheating seas. When coral gets too hot, explained our bingo biologist, it is abandoned by its symbiotic best mates, the photosynthesising algae that provide it both with food and its Fauvist colours. If temperatures stay too high too long, this "bleaching" becomes irreversible and the coral dies. .
Wetsuited and booted the following morning, we humped our heavy scuba equipment into the boat for a five-minute chug to the Coral Gardens, one of the island's 14 dive sites. Once at the right spot, we dropped into the water, accompanied by Filsa, our qualified guide. I ran through the dive plan one last time with the children - stay between me and the instructor, watch your buoyancy, no annoying your brother - and then down we went.
It was a revelation. The waters were so thick with life it was like swimming through fish soup, a veritable bouillabaisse of species swarming about the great bulging heads of coral, all of it vivid with the lysergic colours - tangerine, crimson, saffron - of some 1960s music archive footage. Within seconds of submerging, Jack came face to face with a (harmless) reef shark; minutes later we witnessed a stately fly-past by a pair of giant manta rays. As we hit 11 metres, we hovered above a stingray as it flip-flapped itself into the sandy bottom next to a resting leatherback turtle.
When we broke the surface, 30 psychedelic minutes later, Tom pulled the regulator out of his mouth, and swivelled towards me. "Wow, Mummy," he said. "It's so much better in real life."
Indeed it is. So I only hope his children get the chance to find a Nemo - at somewhere other than the local multiplex.
THE COMPACT GUIDE
HOW TO GET THERE:
Accommodation at Lady Elliot Island Eco Resort (00 617 4156 4444; ladyelliot.com.au) includes tents, cabins and rooms and starts at £50 per adult and £35 per child (aged four-11), half-board.
Australian Tourist Commission (0906 863 3235; calls cost 60p per minute).