The first thing I see when I plunge my head underwater is my own slightly startled reflection.
I begin swimming, gingerly at first, but as the current reaches its full strength I stretch out my stroke, confident at least that I won't end up swimming into the wall. In fact, as I work harder and harder simply to stay in the same place, I begin to have visions of being pinned to the back wall by the force of the water, like a flailing insect.
Of course, this is no ordinary pool. Only 15ft long, the so-called "Endless Pool" is equipped with an adjustable jet stream that enables you to swim normally, but on the spot – like a treadmill. It also features video cameras above and below the water to help assess and improve your technique.
"People often say that pro swimmers make swimming look effortless," says my instructor, Ray Gibbs. "It's because they aren't fighting against the water. Everything they do helps them to move forward – they don't waste energy moving upwards, downwards or sideways."
I'd secretly been thinking that I looked quite professional, but my self-satisfaction disappears in a stream of bubbles as Gibbs stops me, over and over, to play back the footage.
Seeing my faults from every angle is definitely the first step to correcting them. The second is practising the drills that Gibbs prescribes, one of which is called "puppy paddle". "Drills are essential for anyone who wants to improve," he says. I'm hoping to progress to doggy paddle at my next session ...