IT WAS A WINTERY Wednesday night in the basement of a London student's union when I first heard whispers of an island swarming with hammerhead sharks. I had just completed my final BSAC Novice Diver pool session and, while de-rigging my gear, I overheard two seasoned divers talking animatedly about an island off Costa Rica.
“There are like hundreds of them…” one diver said.
“And whale sharks too, don’t forget about them,” the other interrupted.
I remember sighing wistfully.
The following weekend I was heading to Stoney Cove for my open-water training dives, and I’d be lucky to see a pike or perch, or even a hand in front of my face.
One day, I dreamt, one day I WILL get to this magical place.
Fast-forward 20 years and that day has finally arrived. I’m on my way – and the way is long. Very long, 36 hours-across-bumpy-seas long, in fact. As we board Aggressor’s Okeanos I, our dive vessel for the next eight days, Captain Alberto promises us “rain, wind, rough seas”.
And we cheer, kind of.
It’s June, the start of the rainy season, perfect for schooling hammerhead sharks, which are attracted by the abundance of nutrients in the water. But it also means strong currents and stormy weather. Seasickness tablets, anyone?
Three hundred miles later, it’s 6am as the lush volcanic island of Cocos looms before us. Coconut trees cling precariously to its sides and waterfalls flow from rugged cliff-tops, dropping 50m to the shoreline.
“Breathtaking” is an understatement. Not surprisingly, in the late 1990s UNESCO declared the island a World Heritage Site, but the true treasures are to be found beneath the surface.
If it’s pretty corals and relaxing diving you’re after, Cocos is definitely not for you. But if the rush of fast currents gets you smiling, and the promise of big pelagics such as tiger, hammerhead and Galapagos sharks, rays, dolphins and whale sharks gets your heart pumping, read on.

MANUELITA DEEP
Our first day brings with it a signature Cocos dive-site that we (delightedly) will get to dive several times. It’s a 150m-long islet close to where the boat anchors in Chatham Bay. As we descend its deeper west side to 30m, we’re greeted by a large school of blue and gold snapper.
But there’s no hanging around. Our first stop is at one of three cleaning stations in the hope of spotting hammerheads. Apart from a few shadows the sharks remain elusive, so we scoot over to cleaning station number two. On the way an eagle ray cruises past and a keen-eyed diver points upward. I twist my head in time to see a Galapagos shark coursing past in the shallows.
At the third cleaning-station we hug the rocks and wait. The usual suspects flit around – Moorish idols, parrotfish and butterflyfish – before an unexpected school of large marble rays swims by.
Finally our patience is rewarded as four scalloped hammerheads swim out of the blue. They’re bigger than I expected, and slow enough to allow us to grab a few photos.
We watch with glee until it’s time to descend.
At 10m, as we look out into the murky distance, a few white spots flash at us. It’s our first whale-shark sighting. The visibility is poor, however, so only a few of us see it whoosh past.
We get back to the main vessel and shout: “Whale shark, whale shark!”, while the divers on the other dinghy retaliate with: “Tiger shark, tiger shark!”. This friendly repartee will continue after each dive.
One of my favourite aspects of liveaboard diving is not just being able to maximise time spent under water but meeting new dive-buddies from around the world. The sense of camaraderie is rarely experienced in normal life.
Oh, and the food. I lost count of how many times I overheard passengers saying things like: “How do they make such great food in such a small kitchen?”

PUNTA MARIA
At another of the more-challenging dive-sites, usually because of strong currents, we descend 25m down a line to the top of an underwater mountain. The current drops off but, sadly, so has the visibility.
We swim over to a flat rocky area and peer 10m below at a natural amphitheatre, surrounded by large boulders and a sloping wall.
A large Galapagos shark cruises past a few metres in front, then something else catches my eye – it's a free-swimming moray eel sliding along the rock beside me.
Next to my gloved hands, blennies poke their heads out of the barnacles, and red snapper dart in between the rocks.
We fin back to the line, and a school of butterflyfish swarm around us before another Galapagos shark shows up. It turns and saunters in front of us, as if parading down a catwalk. I suspect that with more current we would have seen more sharks, but them’s the breaks.
At least the sun is shining all the way back to the Okeanos I, just in time for morning tea (fruit platter and cake, yum).

SUBMERGED ROCK
The dinghy takes almost 30 minutes to get to this site but the scenery around the island is beautiful, so nobody cares.
As the name suggests, the top of the rock is below the waterline, at 7m. One side is a series of large boulders that tiers downwards, and the other is a steep wall.
Visibility is only 5-10m today, so we follow close to the captain and head to 20m in search of the archway.
The entrance is obscured by a huge school of yellow-lined snapper acting like gatekeepers.
We swim through, one by one, with a short stop in the middle to peek at whitetips resting on the sandy bottom.
Regrouping, we fin with the wall on our left. My buddy spots an octopus sitting wedged inside a hole. We push through the current and continue around the wall, ascending slowly up a large crack in the rock face. Eight rock lobsters are using it as a hide-out.
Then it’s time to float up to 5m and do our safety stop on the top of the rock, in the surge, next to a school of Pacific creole fish.

BEST NIGHT DIVE EVER!
It really is the night-dive to end all night-dives, if you get a thrill from watching whitetip sharks swarm beneath you, around you and even through your legs.
We do three night dives altogther – Chatham Bay, Ulloa and Manuelita Shallow, and boy, did the sharks put on a show! Descending to an average depth of 15m, we had one job – to shine lights for the hunting sharks.
At the end of one dive I hung back, and the scene below reminded me of the M4 and M25 junction, only with sharks, instead of cars, zipping across the lanes.
But the true heart-racing moment is when the feeding frenzy begins, and then all bets are off if you’re a fish. The sharks go nuts. Some of the braver divers creep into the fray, while others hover above, happy to watch from a distance.
I’ve done a few hundred night-dives but never experienced anything like this one before. It’s like living in a BBC wildlife documentary, with the bonus of hot chocolate and cafe crème liqueur waiting for me when I get back on board.
Can a night be any more perfect? Oh wait, yes, take a glass of wine to the top deck, then stare at the Milky Way, shining in all its unpolluted glory.

DIRTY ROCK
Leaving the best until last, it’s a short dinghy ride to the dive-site of Dirty Rock. The captain spins the boat to the ocean side of the rock and confirms that there’s no current. We fin down to a sandy patch shining below us at 34m.
As we get closer I spot whitetip sharks lying on the sand around a small concrete block with a rope attached (research equipment set up by the rangers).
This can mean only one thing – this area must see a lot of shark action. Whoohoo! We snuggle into the rock and wait. A marble ray floats by, but all eyes are on the block and rope.
It’s not long before a patch of light grey cruises in, a hammerhead shark. Three more follow. The sun filters through the blue hue and increases the visibility to 15-20m, and a few more sharks glide in.
Eventually they disperse and, as I swim on, I peer upwards. The outline of the reef reminds me of the jagged peaks of the Scottish Highlands.
I count the number of whitetip sharks that swim past us – at least eight of varying sizes. I remember the captain's first briefing, when he told us: “There are many whitetips here, and after the first dive you’ll be like “oh, not another whitetip!” He’s not wrong.
Other marine life includes spotted pufferfish, yellow trumpetfish, large wrasse, moray eels and spiny lobsters.
When our guide makes a rattling sound we follow his hand and discover five hammerhead sharks schooling above us between 10 and 15m.
We creep closer to the rocks and observe. When our computers force us to ascend shallower, the sharks disappear.
We reach the end of the rock and swim through a natural channel, which dips to 40m and beyond. Five metres away is a smaller pinnacle. The current is racing, so we grip the rock and watch a huge school of jack dance in the rush of the water flow.
On the final dive of our trip, at Dirty Rock, we’re at 5m and muffled shouts and frantic pointing reveal a squadron of hammerheads schooling beneath us, riding the currents out into the blue. Ah, what a dream end to a perfect trip!

AGGRESSOR FLEET HAS two boats that travel to Cocos, both named after the god of the outer sea that circles the Earth – Okeanos I & II. Most trips last 10 days but some, like mine, take eight.
We did three or four dives a day, depending on what the park rangers agreed on. We carried out 17 in total, at nine of the 20 sites, including the three night dives. All diving was off RIBs, usually with a negative back-roll entry because of the usually strong surface currents.
The weather gods were kind, however, and we had unbelievably good weather – mostly sunshine and relatively smooth ocean conditions.
Nitrox is available and highly recommended, because most of the shark action is between 20 and 30m. Carrying a DSMB is mandatory.
Maximum dive time is 55 minutes and night-dives 45, though we stretched this a little because the dives were that awesome, and the crew didn’t seem to mind. They want you to experience the best diving Cocos can deliver.
It’s not surprising that Cocos is named one of the top dive-sites in the world by PADI, or that Jacques Cousteau described it as: “The most beautiful island in the world”. Or even that Michael Crichton based his Jurassic Park novels there.
Like Galapagos, Cocos is one of those rare places still relatively untouched by the outside world, both above and below the waterline. Even those who suffered seasickness on the 36-hour crossing said they wouldn’t hesitate to come back.
Me? I won’t be waiting another 20 years to return, that’s for sure.

FACTFILE:
GETTING THERE: Fly with British Airways and American Airlines to Costa Rica capital San Jose.
DIVING & ACCOMODATION Okeanos I is a 34m boat that can carry 22 passengers and has a crew of eight “superbly professional” crew, says Marie. She adds that it is exceptionally comfortable, the spacious dive- and sun-decks especially so. Lunch and dinner is mainly buffet-style, except for barbecue night and a set four-course-meal night. The boat picks up from two Holiday Inns near San Jose, aggressor.com
WHEN TO GO: Okeanos I & II dive Cocos year-round. The best shark action is in the rainy season from June to December, especially July/August. January to May (the dry season) is warmer, has calmer seas and good visibility but fewer sharks. Water temperatures is 24-30°C. A 3-5mm suit is recommended, but Marie wore a 7mm and hood!
CURRENCY: US dollars.
PRICES: Return flights from London Gatwick to San Jose from £630. Eight-day trip on Okeanos I are priced from US $4099, and 10-day trips from $4899 (plus park fees and emergency evacuation plan). Nitrox costs $100 for eight days, $150 for 10 days. The Holiday Inns charge $60-90 – cheaper 3* hotels are available at £30-50 a night.
VISITOR INFORMATION: visitcostarica.com