Although he is in full possession of his faculties and all limbs, Dr Mark Erdmann is about to flush a shark from its hidey hole.
"Watch this," he says, and his fins arc into the sky as he duck-dives and swims to a ledge about four metres down while I hover on the surface. His hand disappears into darkness and a mottled, moth-coloured, two-metre shark darts out and stops on the bottom about 10m away.
It's a wobbegong, a carpet shark. Not especially fearsome, certainly not aggressive, but a shark all the same. Provoked, a wobbegong will bite, and when they latch on they don't let go. Now Mark is back on the surface, grinning.
"See that? You can stroke them," he says, and before you can say "double amputee" he's fondling the docile shark, scratching it just behind the dorsal fin.
It's been a startling encounter, but only par for recent experience. I'm on a 12-night cruise through the islands off the coast of Indonesia's West Papua province.
From Sorong, on the north-west coast of the Papuan bird's head, we travelled north-west through the Raja Ampat islands, crossed the equator then turned south through the Maluku Islands, formerly the Moluccas, the original Spice Islands, before the finish line at Ambon.
Mark Erdmann, our expert guide, is a marine biologist and head of the Bird's Head Seascape programme for Conservation International, which has established several marine protection zones in the region.
Over the past week, against hedges of pink and purple corals sprigged with sea fans, we've swum with manta rays, hawksbill turtles, seahorses and moray eels and watched lionfish puff up their poisonous spines. We've been pounded by chilly waterfalls where they cascade through rainforest, swum in an evening sea while stingless jellyfish drifted overhead like moons, swum through caves filled with squeaking bats and risen before dawn to creep through a wakening forest to watch a male red bird of paradise at his mating dance.
Several times we call in at villages, where the villagers dress up in grass skirts and feathers, arm themselves with swords, dance, bang drums, lay out feasts of mud crabs and make speeches, all in our honour. In an area where cruise vessels are unknown, we are feted like royalty.
These islands are toothy constructions of limestone that murder bare feet. Sheer-sided and generously greened, they soar straight from the sea, covered with thick rainforest and karst palms and garnished with orchids.
Beaches are almost non-existent. Where the islands meet the water they are ringbarked by the waves, which leaves the smaller islands looking like green muffins.
In contrast to the 100 million-plus people who are squashed into Indonesia's island of Java, this is Robinson Crusoe territory. Most days we see a handful of fishermen in outrigger boats, but villages are almost non-existent.
Only a few intrepid outsiders visit these waters, mostly on live-aboard dive boats, and what those few discover is something very special because what lies below the surface in these waters is miraculous.
These waters are at the absolute pinnacle of underwater biodiversity. The 2 500 islands and reefs of the Bird's Head Seascape have nearly 1 300 fish species, 600 coral species and 700 molluscs. Unknown to marine biologists until the 1990s, it's blown all previous counts of marine life out of the water. Surveys in 2006 revealed at least 56 previously unknown species.
In a single dive in the Raja Ampats, the prominent Australian ichthyologist Gerry Allen counted 283 fish species. Against this, the Caribbean, the Red Sea and even the mighty Great Barrier Reef look positively pale. "It's a species factory," Mark Erdmann says. "A large number of reef fishes are known only from this area."
As if to underline this judgement, when we dive off a fresh lava flow off the Gunung Api volcano, he discovers a new species of dottyback.
Our vessel is the True North, 50m of crisply tailored luxury and just about the last word in expeditionary cruising.
Fully air-conditioned, the 36-passenger ship has a large lounge/library with a bar, a smart dining room, Internet facility and flat-screen TVs with VCRs in the cabins.
There's also a forward observation lounge and access to the forward deck, plus a few accoutrements you won't find on just about any other expeditionary vessel, such as the Bell 407 helicopter on the top deck.
It's not a voyage for those who prefer to watch the scenery unfurl from a deckchair. You're signed on for adventure, as conceived and choreographed by owner Craig Howson, and that means plenty of everything - snorkelling trips at least three times per day, heaps of fishing, walks at every opportunity, the option of helicopter flights over coral-rimmed islands, a few late nights and plenty of laughs. Breakfast is at 6.30 every morning and activities begin half an hour later.
Underwater, it's another kind of feast.
We drift above damsel fish and giant clams, watch cleaner wrasse busily cleaning gills, play with clownfish and hover above feeding manta rays, which are possibly the most elegant fish in the ocean. If Dior created a fish, it would be the manta ray.
The tints in the corals ridicule rainbows, and every reef is another world. Many of the areas where we snorkel are marine protection zones and the fish are sociable. Mass shoals of fusiliers glide within centimetres of our fingers.
It feels as if we are the curiosity, the intruder as spectacle. Everyone who snorkels comes back grinning, even those who must be forcibly extracted from the water.
Overwhelmingly, the passengers sailing aboard True North are baby boomer couples with deep pockets, a taste for adventure and the sort of wristwear that can get you into a first-class airport lounge. Most have travelled aboard True North before. The crew could well be their children - energetic and efficient 20-somethings with some surprising talents.
Richard and Luke turn out bistro-smart meals for lunch and dinner, and treats several times a day. Jarred can talk through his snorkel, and often does. Frank can free-dive to 20m to retrieve lost equipment, Mikey teaches me a few Photoshop tricks and Mark Erdmann takes Denise up and over a personal boundary when he talks her into leaping 10m from a limestone cliff into a river below.
For me it's scuba diving. It's been more than five years since I've last strapped on an iron lung after a terrifying dive in southern Australia. I thought I might never dive again but gently and irresistibly Kay persuades me, and before I know it I'm 27m down, consorting with enormous bumphead wrasse, pipefish, batfish and rays.
Whether you're a fish or just pretending to be, it's an amazing place.
- The next West Papua Discoverer cruise aboard True North will take place on October 18-30. For more information, see www.northstarcruises.com.au