Most new mothers would take it easy on a Greek island holiday, but for Mary Loudon the challenge of the Ionian Sea was irresistible

They say there are no sharks around the Greek islands of Ithaca and Cephalonia, but I know that's not true. I have seen a soap shark in Cephalonia, in Sami harbour, and it was approximately two and a half metres long. Of course, soap sharks are harmless to humans, and more scared of us than we are of them. But I didn't want to meet one when I was in the water myself.

They say there are no sharks around the Greek islands of Ithaca and Cephalonia, but I know that's not true. I have seen a soap shark in Cephalonia, in Sami harbour, and it was approximately two and a half metres long. Of course, soap sharks are harmless to humans, and more scared of us than we are of them. But I didn't want to meet one when I was in the water myself.

So it was partly a visceral fear of the deep, of the possible great unknowns lurking beneath, which naturally translates itself into a fear of something identifiable, like sharks. And it was partly the idea of getting there entirely under my own steam, a tiny dot of human being in that wet wilderness. But really, it was the sheer beauty of the endeavour that made me decide to swim from the beach-garden of the villa in Fiscardo, Cephalonia, where we were staying, across the sea to the island of Ithaca, misty blue with distance, mysteriously indistinct.

I am obsessive about swimming. I swim nearly every day, a mile or so, in my local pool, something I've done since my teens. My aquatic background was a privileged one. I was lucky enough to grow up with a broad stream running through the garden, which was deep enough for children to swim in. There was also a good local swimming-pool, plus two school pools that we were allowed to use, one of them outdoors. Such riches seem to me now almost an outrage.

On family holidays, I pestered constantly about being able to swim. It didn't matter what type of water, or where, or what temperature. I swam in the sea, rivers, lakes, pools, and freezing cold mountain streams. I even threw myself, completely unbidden, into a couple of filthy canals: the first time, I was severely reprimanded; the second, violently sick. But like a dog that sees water and barks for a stick to be thrown into it, I cannot let water go unremarked upon, cannot see it without experiencing instant longing.

Last October, contemplating Ithaca with my three-month-old daughter in my arms, I considered my desire to slip into the waves beneath the bedroom balcony. It had a lot to do with the physical and mental challenge, with being something insignificant in an overwhelming environment; not so far off the beaten track from the delights and demands of new motherhood. It had also to do with adventure. Unlike the environments of the walker, runner, biker, or skier, a distance-swimmer's terrain is mostly indoors, confined within an unremitting cycle of tumble turns. The Ionian Sea is a far cry from your average UK leisure-centre pool. The Greek islands themselves, like the Alps or the Bahamas, have a resolute identifying image: they are understood in a kind of shorthand of golden sand and azure seas. A journey of my own making across a stretch of water in such paradise, and possibly uncharted by other swimmers, was a thrilling prospect.

We were on holiday in Cephalonia because we have friends who live there. "You're crazy," says Angelo, when I tell him what I want to do.

"Are you worried about the wind?" I ask.

The afternoon winds are notorious around Cephalonia. After midday, the sea chops up as if someone were whisking it from above, and it is suddenly easy to understand why the ancient Greeks believed in the wrath of the sea gods. In the afternoon, the Ionian lends itself naturally to a belief in Creation's reward and punishment. In the morning, just before sunrise, the wind drops and the water reassumes a look of blank innocence.

"Forget the wind," says Angelo. "There are terrible currents out there. It's a shipping channel. There are tankers. Really, this is a crazy thing. Dangerous. Don't go alone."

Angelo and his wife Sophie have spent many years sailing. They live on a boat half the time. Angelo knows what he is talking about. And I have no intention of going alone, though he doesn't know that yet.

"Come on, Angelo," I say. "Come with me in your dinghy. We'll go in the morning, before it gets rough."

"Swim to Ithaca? You're crazy," says Angelo. "My God, it's over two land miles."

A couple of rounds at the leisure centre.

"You could go fishing," I suggest.

"OK," says Angelo. "So it's a fishing trip. Now you're talking."

It's actually very easy to let go of land, and the sensation of being utterly released from it increases the further out to sea you swim, a bit like watching countryside recede from the window of a climbing plane. When pebbles and sand gave way to thick, greeny-grey weeds, and finally to nothing but penetrating blue, I turned to wave at my husband and daughter watching from the villa balcony, but could see them no longer. There were no fish to look at this far out, either, just the sunlight reflected and refracted back upwards, in millions of silvery shards. I was wearing a snorkel and mask, in order to swim most efficiently, and so as not to choke on the waves. This meant that I had my head in the water almost the whole time, which was both a good and bad place for it.

Good was the meditative concentration it afforded, the entire giving of oneself to physicality; for me, it meant reinhabiting my body in the only way that I could fully understand after weeks of round-the-clock feeding and waking up for a baby ­ arms and legs in rhythm, limbs stretched and taut but nearly weightless.

Bad was thinking soap shark, or octopus. The previous day, I had spied an octopus wound around the anchor chain of a small boat. It was so enormous that I persuaded myself for one insane moment of panic that it was a drowned anaconda.

From time to time, I raised my head to see where I was going. I was heading for a particular rock, a golden triangle rising from the water, and taking guesses as to its actual height. I had no sense whatever of the current that swept me nearly a diagonal mile from my chosen route, and it was only when I raised my head that I had any sensation of the tide, even though the swells of the large tankers passing between islands create waves on the beaches. This surprised me because, frankly, I had been expecting a bit of a fight. But conflict only happens where water meets obstacles. I was no obstacle, I was now a mere particle of the sea itself, subsumed. It was a wonderful feeling, the water was warm and extremely buoyant, and the swimming was so easy it was laughable. It was the most perfect swim of my life, my body and spirit held in suspension, thoroughly alive, yet disengaged.

I engaged pretty swiftly when I looked up to see a large tanker bearing down on me. "Keep going," called Angelo, "it's half a mile from you."

You could have fooled me, but actually he was right, though for some time after it passed, the swell was so high that I couldn't see the dinghy, affording me an insight into the acute loneliness of the long-distance swimmer.

The last half-mile was the oddest, because the land, while growing closer, seemed always to be moving out of reach, assuming the impossibility of a rainbow. And then my triangular rock, which was not my intended rock at all, but another like it, began to rise from the water. And as it grew larger, and the seabed reappeared in glorious gold and turquoise, ground rising from below and towering above, I felt the land closing its jaws. I reached the rock, slapped it hard, and turned back towards the boat, reminding myself that petty victory pales besides the liberation of belonging for an hour and 20 minutes in a world that does not belong to you.

Mary Loudon is the author of 'Secrets & Lives, Middle England Revealed' (Macmillan)