Somewhere around us in the incredible turquoise and blue-black waters of the Maldives, the planet's biggest fish is swimming by.
Reaching lengths of up to 20m and sporting a dramatic checkerboard pattern of bright polka dots, you'd think that spotting a whale shark would be easy.
But we've been peering into the water for three hours now and so far, nothing.
We're cruising up and down a known shark aggregation zone, a stretch of the Indian Ocean outside the island necklace of South Ari atoll, one of 26 coral formations that make up the Maldives archipelago.
On board are conservation biologists Richard Rees and Adam Harman from the Maldives Whale Shark Research Programme and the tagging expert accompanying them, Brent Stewart, of Hubbs-SeaWorld Research Institute in San Diego, US.
Brent has tagged everything from seals to sea birds to learn more about their lives, and he's also tagged a group of whale sharks off Kenya.
This project, which began last year, is the first attempt to tag whale sharks in the Maldives and the team is hoping it will reveal precious information about the little-studied fish.
Spot the difference
Whale sharks ( Rhincodon typus ), first discovered in the 1800s, are found throughout the tropical oceans, but relatively little is known about their behaviour, how long they live, their breeding habits, or their migratory routes - or indeed whether they migrate.
The group is keeping an open-access database of the sharks. This means that each time one is found, one of the researchers will free-dive down to take a picture of it between the fifth gill and the side fin on both left and right sides.
Using software similar to fingerprint-matching technology, the snaps of the shark's spot patterns are compared to see if it has previously been photographed or is a new find. So far they have recorded 106 on the database, all but two of which are male.
But before they can be snapped, we need to find them, and we're not the only ones searching. Speedboats frequently whiz past us carrying resort tourists on whale shark-spotting trips.
The activity is increasingly popular and has become big business here, which Richard thinks may be related to the number of boat impact injuries sustained by the sharks.
In a survey Richard's team recently completed, more than 75% of whale sharks recorded in the area had scars or wounds from boat damage. They are especially vulnerable because they swim slowly and close to the surface.
His organisation is campaigning for a large Marine Protected Area to be established here to protect the sharks from boat damage and fin-poaching.
While the others keep a lookout for sharks, Brent talks me through the tagging procedure and shows me some of the formidable equipment they'll be using, including a harpoon and a spear gun.
The sharks are tagged just beneath the dorsal fin into a thick layer of fat - they rarely even bleed. The group is using two types of tag, one of which is programmed to release after a few days.
This records data including temperature, depth and light level, before bobbing to the surface where it will transmit its data via Argos satellites.
The other, smaller, type of tag stays on the shark indefinitely and needs to be cut off at a later date by researchers.
Brent is also collecting skin samples from the fish for mitochondrial DNA analysis and other studies. This, he hopes, will help to reveal how closely related the sharks in the Maldives are to each other and to sharks elsewhere in the world.
Some sharks he has seen have such similar spot patterns that Brent thinks they must be brothers, but nobody knows for sure.
He is just finishing how the equipment works when Adam finally spots a whale shark. There is a brief moment of procrastination - other calls have turned out to be shark-shaped coral blocks or manta rays - but then everyone springs into action.
Masks, snorkel and fins on, I leap into the water after the researchers and swim my fastest towards the large shark that's drifting along, feeding on plankton.
Adam passes me one end of the measuring rope and I power up to the shark's nose, looking directly into its eyes as Adam stays at the tail. The shark is a youngster, just 6.5m long.
Meanwhile, Richard has swum beneath the fish to sex it; small claspers confirm that it's a male. He snaps the shark on the left and right side with an underwater camera. Brent swoops in with the harpoon and tags the shark.
We are surfacing when Richard spots another and we free-dive down again. This poor creature has a badly distorted dorsal fin that is almost completely detached.
Back on the boat, the team explain the sad story behind "Joey's" fin.
He was first photographed by the group in 2007 in perfect health. Then, one night last year, they got a call from people on a local island saying that there was an injured whale shark floating in the island harbour.
Arriving at the scene, Richard saw that it was Joey, who had suffered an unsuccessful finning attempt - his dorsal fin was very nearly severed, left hanging on by a small segment.
"It was a terrible injury, we thought he probably wouldn't survive," Richard says. But in time the wound healed and Joey is still swimming around.
A shark fin of this size can go for $10,000 (£6,600) in Taiwan or Hong Kong, and can be used as an eye-catching billboard outside a restaurant serving shark fin soup.
Joey's story is sobering, despite our euphoria over being lucky enough to spot these incredible creatures. Luckily for them, the new Maldivian government is beginning to take shark welfare seriously and has introduced a reef shark hunting ban throughout the 26 atolls.
Whale sharks are described as "vulnerable" on the International Union for Conservation of Nature's Red List, with their population expected to decline by as much as 50% over the next century.
But in truth, nobody knows how vulnerable they are, the true number of whale sharks in the world, or whether that number is in decline or increasing.
We don't even know why the team has only seen two females and more than 100 juvenile males in the Maldives.
Where are all the other females, and where are the adults? These are some of the questions the team hopes to answer over the coming months in collaboration with other research groups. The organisation is also trying to set up a permanent research station in the atoll, to compile year-round data on the animals.
We spot three more sharks later in the day, one of which the software reveals is previously unrecorded. It is a male, number 106, and the team names him Nick, in honour of our cameraman Nick Pattinson. He is fantastically pleased and we all return to harbour delighted after our encounters with the world's largest fish.