I am not a naturally brave man. I don't, as a general rule, go out of my way to put myself in dangerous situations – or even ones that I'll find very uncomfortable. But I rarely refuse a dare. That's how I've found myself bungy jumping or skydiving despite a ridiculous fear of heights, or ingesting suspicious-looking cheese.

And that's how I found myself face-to-face with one of the terrors of the ocean: Carcharodon carcharias, also known as the great white shark.

A friend of mine – let's call her Jane (though obviously that's not actually her name) – is a marine biologist. A behavioral ecologist, actually, who works on sharks. She studies their movements, their diets, their life cycles, their interactions with humans, that kinda thing. She often goes diving in shark-infested waters, which constantly amazes me because she's a tiny accident-prone girl who can barely go a week without walking into a wall or burning her hand on a stove or something like that.

She dared me to accompany her on one of her shark-tagging missions. So obviously, I accepted.

The great white shark (GWS to its friends) is so named because it displays strong countershading: its bottom side is white so that creatures looking up won't notice it against the sunlight from above. Its top is grey so that it blends with the sea floor when viewed from above. This pattern also breaks up the outline from any angle, and the mottling helps. Sharks are magnificent creatures, shaped by millions of years of natural selection for marine predator efficiency to be streamlined killing machines. They've been known to reach 6m in length and exceed 2 tons in weight. They eat a whole variety of fish and mammals and seabirds and pretty much anything they can get their mouths on, really (metaphorically speaking). Add to that they lead the shark pack in their number of attacks on humans. They have rows of serrated teeth behind their already intimidating main set of teeth. They lunge way up above the surface to catch their prey. It's no wonder most people avoid them at all costs.

Great white sharks are found all over the world where coastal and offshore water temperatures settle around 10 to 20 degrees C. But they're especially common around South Africa...which is why most shark research happens in my backyard.

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So I've accepted the dare, and I'm in Gansbaai with Jane. Gansbaai (Afrikaans for “Bay of Geese”, don't ask) is a little fishing village in the Overberg District Municipality of the Western Cape with a super-dense population of great whites – and naturally, the parasites that go along with them, shark cage divers, shark tour guides, that kinda thing. It's a popular shark tourism destination, basically (and the whale watching doesn't hurt) a good few hours' drive from Cape Town. It's a drive that's particularly beautiful on a sunny summer morning. Honestly, it does a body good to get away from the big city for a while, and the mountain pass is just so tranquil – at least when you're on a stretch of road that's not being worked on.

It's early in the morning, and we've stopped at one of the operating outfits (The Great White House, where we've been hanging out with researchers and tourists and Pepe the Mexican, who loves tequila, and yes, I'm being absolutely serious). Jane has arranged our boat and our skipper, a long-time friend of hers whose name is not Skip – but for the purposes of this story, let's say it is. He's friendly, calm and confident, and looks like he knows what he's doing. He and Jane have done this for years, and they swiftly walk me through the plan, joking all the while.

The procedure is simple: we sail out to sea with a bucket of chum leaking from the back of the boat, find a good spot, weigh anchor, wait for the sharks, stab them with these really long spear-like things and thus attach radio transmitting tracking devices, test them to see if they're working, and repeat. (“Don't worry,” Jane says, “their skin is so thick they barely feel it, and anyway life in the sea is far too filled with real threats and pain for them to worry about this.”)

So we launch off. The water's choppy, sure, but it's nothing I can't handle. I'm a tough guy, you know. The boat takes off at a swift pace, cutting across the waves, bouncing as it hits them. Within fifteen seconds I discover that there is something I totally wasn't prepared for: sea sickness. Sea sickness doesn't care how tough I think I am.

Sea sickness, you see, is just a special case of motion sickness, also known as travel sickness or kinetosis. Other special cases are air sickness, car sickness, and probably horseback sickness, for all I know. Basically it happens when there's a disagreement between what your visual system is reporting about your movement, and what your vestibular system is reporting about the same. The vestibular system, for those not in the know, is the sensory system that provides the majority of your information about balance and motion. It's inner ear based: the semicircular canal reports rotational movements, and the otoliths handle linear accelerations. So when you feel motion but don't see it, or see it but don't feel it, or when the systems both report motion of very different stripes, you may experience motion sickness: dizziness, fatigue and nausea, mainly, with epic headaches thrown in for good measure. Apparently it's really common, with most people suffering from it under extreme conditions, and as much as a third of humanity suffering it even when there's only mild motion involved.

It is Hell on Earth.

So one minute into the trip and I'm clutching the deck on all fours. This does not help in the slightest, but neither does anything else. Focus on the horizon, they tell me. Or stare down at the boat. Try standing up. Walk across the ship. Pop your ears. None of this makes any difference. I feel lower than I thought it's possible to feel, and I'm not getting any better.

It's going to be a very long day.

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So we've been out for a while. Sharks have been flocking to the boat , and we've moved several times now. Skip and Jane have tagged a whole lot of them. So far I've barely even seen them. I have, however, managed to change the chum bucket a few times. And I've heard a couple touristy shark cage diving ships shouting to us across the way. Who would even try to hold a conversation under these conditions?

Random thoughts pop in and out of my head, and all I can do is watch them go by (which leads to thoughts of Descartes and Hume and Kant and The Matrix sailing along as well). I don't have enough energy to focus on them or tame them or try to organise them.

My sister, too, is a marine biologist (and a damn fine one, I might add). She spends weeks at sea at a time, collecting all sorts of data and she gets seasick. Constantly. If it's anything like this, she is clearly far, far tougher than I give her credit for. I should tell her that sometime.

Just the other day, I watched Jaws for the first time in my adult life, and possibly ever (short review: part terrifying, part hilariously silly, but overall quite entertaining). An entire generation was terrified by that movie – and those guys only had to deal with one shark. There are three around my boat right now (according to Skip and Jane, at least – I can only see two at a time from my vantage point, and they all look pretty similar to me). They're the least of my problems.

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It's turning into a mildly drizzly day. I don't really notice. There's no space in my mind to notice anything beyond the constant blahness. It doesn't help that the wind's picked up a bit, though.

“Are you alright?” Skip asks.

“Hrngh.” That's pretty much all I can manage.

“Don't worry,” Jane says. “We shouldn't be out here too much longer.”

She gives me a sandwich to replace what I've lost.

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Three hours later.

I'm still hanging on for dear life. I've already vomited sixteen times – very neatly, four sets of four retches. What makes this really amazing is that all I've had to eat all day is: one apple; one cup of coffee; one sandwich; and one bag of Jelly Tots (little colourful jelly-sugar sweets that I've loved my whole life, and one bag of which I once kind of paid the equivalent of $70 for, but that's a tale for another day). Where is all this stuff coming from? Did I just puke up a kidney or something?

I've been on this boat for eight hours – 480 minutes, or 28,800 seconds, every one of them unpleasant . I've managed to stab one shark, twice (I thought I might have gotten another one, but it turns out I was just confused, and hey, can you blame me?). I helped pull in the anchor a couple times, but mostly left it up to poor tiny Jane and the doughty Skip, since standing really wasn't much of an option for me.

With a great effort of will, I raise my head and look about. There are indeed three sharks. Any one of them looks big enough to swallow a substantial fraction of my body in one gulp. Seriously, these things are huge. And they're not even particularly big ones, according to Jane. But I don't care about that. I'm weighing my chances of making it to shore if I dive in now and just swim for it.

I used to be a fair swimmer back in the day. But at my fastest, I can swim at maybe 6 km/h, and that's if I jettison my clothes; a great white can move five times faster. Plus, in the water, humans are pretty helpless – we can't see where we're going, we can't generate enough force with a punch to hurt anything, we have to keep coming up for air while most everything else just gets their oxygen right out of the water.

Sharks have a few more advantages: ludicrously keen senses of smell, an ability to maintain a body temperature warmer than the surrounding water even as mine would plunge, and ampullae (or Organs of Lorenzini) which allow them to sense electrical fields produced whenever animals move. They've got me way outclassed. This would not be the most dignified way to die, naked and chowed by a shark. I'd probably get a Darwin Award. It would be insane to try to swim.

I spend ten minutes working through this.

Finally, I drag myself to the side of the boat, near the chum bucket, and look over the side, despite Skip and Jane telling me this is not a good idea.

It's now or never.

And just at that moment, a shark surfaces to chow some chum. It's huge. Not as big as it is in the movies, maybe...but this one is right here. Those teeth look evil. It's whole face is shaped like a killing instrument. It could take my head off easily, and probably unpleasantly.

Then I feel a stirring from within and I vomit.

I vomit. I vomit.

An entire sandwich worth, and this time there's no neatness involved. I just let it all go.

When I'm able to think again, I see that there's no shark. And Skip and Jane are laughing like crazy, actually doubled over with laughter, which is something I've never actually seen before.

“What?” I manage to get out.

“You just looked like you were about to climb out of the boat,” says Skip.

“And you vomited over my favourite girl shark,” adds Jane. “I think you broke her spirit.”

“That's what she gets for preventing me from swimming to a better world,” I retort.

She smiles. “Maybe it's time to go back. We can tag a few more tomorrow, if the weather's better.”

That might just be the best news I've ever gotten. I reluctantly agree (“Are you sure? I could stay a bit longer if you'd like, don't worry about me,” brave words, and not very convincing at this point). And so we draw in the anchor (I help this time; the prospect of an end to my misery gives me new strength) and head to shore.

And thus ends (my version of) the (far-too-often-retold) tale of Nick's Great White Shark Adventure, and how I became The Man Who Vomits In The Face of Danger.

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The rest of the trip was nice, though. Skip and his pals took Jane and me out for dinner at a perfectly pleasant place with fancy food that I actually managed to keep down (a first for the day). Then we hit the pub (adorned with pictures of past party going folks, including an ex-girlfriend of mine) and we had a few drinks and a few laughs (many of them at the expense of the vomiting landlubber who feared no sharks) and a lot of fun, and then we had a few more drinks and some more fun and some more laughs and we called it a night (it was a week day, you know) and I crawled into bed at the guest house and...

And that's where the story ends, thank you very much.

But I'll never forget how close a call it was, that choice between swimming to shore and suffering in the boat. The devil in the deep blue sea can't hold a candle to the demon of disorientation in the inner ear. Remember that next time you laugh at a landlubber, okay?