I'm panicking. Now the time has come to put my big toe in the water, the fear I have been stifling for weeks wells up with full force. Every shadow below the water sets my heart pounding. Pointy white teeth flash in front of my eyes. The theme tune to Jaws drums in my ears: der... dum... der... dum.
I'm shark diving. It's the latest adrenaline sport and one that is quite simple: just find a patch of sea with plenty of sharks lurking below, then jump in. Except in this instance I'm at an aquarium in Scotland, working up to the open-water experience with a new shark-diving course at Deep Sea World, in Edinburgh.
Pitched as a confidence builder, the day introduces divers to shark biology, conservation and behaviour.
But it isn't a walk in the park. Circling the depths below me are five sand tiger sharks (dangerous if hungry), a few stingrays (dangerous if stepped on), a couple of conger eels (dangerous if frightened) and several other murderous toothy and tentacled creatures.
The trickiest aspect to diving with sharks is getting into the water in the first place. Our group of 10 spent the morning in a classroom hearing how it's safe, how sharks generally avoid divers, how they don't like the taste of Neoprene wetsuit, how air cylinders play havoc with their buoyancy, and how we're more likely to get bitten in New York than in the sea. But no matter how many times you are told that sharks are not bent on your untimely death, a primitive but sensible instinct located deep in your brain stem prevents you from taking the plunge.
Grant, our tutor, gives a lesson on the right way to do it. It's very simple, he says. Wear muted colours and take off your jewellery ("It glints, like fish"). Slip in slowly and avoid splashing. Get under the water as quickly as possible ("On the surface, you look like a seal"), then once you're safely below, stay in a group and, if a shark approaches, back off slowly. "Don't bolt for the surface - and don't chase or touch them," he adds. As if.
Lecture over, we pull on drysuits, masks and boots rather than fins to avoid disturbing the marine life. By now, I'm cotton-mouthed and my internal monologue is a mishmash of muddled instructions punctuated by XX-rated expletives. Der... dum... der... dum.
Once submerged, we lunge in slow motion like spacemen along a sandy floor until we reach the shark-watching area. It is a surreal experience. The aquarium has been designed to simulate a real underwater environment, and en route we pass a shipwreck, a kelp forest and an abyss (which I'm tempted to escape down). We are watched all the time by friends and family who have gathered in a perspex viewing tunnel, clearly concerned that we are about to become lunch.
There's just enough time to dodge an incoming ray before our first shark rounds the corner. Tinkerbell, a 10ft sand tiger, is all teeth and eyes, with no discernible fairy-like qualities whatsoever.
We have been promised that she is docile unless provoked and, with no intention of upsetting her tranquillity, we let her weave between us while we stand in the water, frozen with a mixture of fear and fascination.
She swims so close to me that I can see my own reflection in her slate-grey eyes. In the curious distortion of time that often accompanies a particularly intense experience, I find myself peering into her open mouth for what seems like hours, fixated on her pristine dental work and the sheer size of her jaw. Tinkerbell is beautiful; menacing and terrifying - but beautiful. Her skin is as smooth as silk; her streamlined fins an engineering marvel. She has the bulk of a bull, but the grace of a ballet dancer.
After this, it's a doddle. During the next half-hour, we survive several close encounters with sharks and by the end, I feel so at ease with Tinkerbell and her teeth that I'm almost - almost - tempted to reach out and stroke her.