Beneath me is the deep blue abyss of the Aegean. Eyes, shoulders and mouth are respectively stinging, chafing and dried by the salt. But I’m into my rhythm now, stretching out my newly improved stroke, pulling my arms through the water until the tips of my fingers brush my thighs.

Every so often, I can make out the island of Schinoussa and the white houses on a high plateau that we are to aim for. I wonder where the others are. My fellow green caps and I set off together, but now I’m alone. I don’t look too hard – we know where we’re heading, and there are two rescue boats to look out for us. In this warm but not enervating water, propelling myself from island to island, I feel a sense of purpose and calm.

My girlfriend, Jeanette, and I have signed up with Swimtrek, a unique travel company that specialises in swimming holidays around the world. The list of destinations is varied and growing: Croatia, the Bavarian lakes, the Inner Hebrides, Turkey and the Virgin Islands, among others. But for us, there was only one choice – the Cyclades, literally “the circling”, a cluster of two dozen Greek islands in the heart of the Aegean.

The concept is that you swim from island to island, accompanied by safety boats, making your way around the circle. Of course, holiday logistics mean it’s not quite that simple – in order to avoid moving hotel every night, you often hitch a lift back to the day’s start point on the company’s yacht – but the fact remains that you’ll be in the water for two or three hours a day. It’s both your raison d’être and your means of transport.

We start on the island of Antiparos. Soon after we’ve met up with the two guides and our nine fellow swimmers at our hotel, we’re being frogmarched to the beach, where, Speedos, hats and goggles in place, we are asked to swim out to the headland and back. It’s a chance for the guides to measure our different abilities.

For the past three months, we’ve been keeping to a training schedule. But an indoor pool, or even a cold lido, is a far cry from the Aegean. We wade out through the warm shallows: the water temperature here allows Swimtrek to run the trip from the middle of May until the end of October.

Now, on the last day of June, it is already 26C. As we begin swimming, I notice just how salty the water is, far more so than the Atlantic. THAT EVENING, we are briefed and handed different-coloured swimming caps – yellow for the “scenic swimmers”, green for the middle group, which includes Jeanette and me, and pink for the three women who are way out in front.

At dinner the first night, the talk is of triathlons, vegetarianism and the perils of drinking too much. Later in the room, contemplating a 7.30 swim the next morning, we wonder if we’ve come on the right trip. It all seems a tad wholesome. Have we ended up on a holiday for health freaks?

We awake to strong winds. The yellow caps are to forgo the crossing, but green and pink caps will do it. After all, it’s only a 1.5km loosener from Antiparos to Paros.

It feels a bit surreal to be walking down to the little port in your Speedos at this hour on a Sunday morning. In the water, everything seems normal as we head out through the boats of the little harbour to the point. Then, past the headland, the waves hit us.

With spray all around, I fix on the sea bed to measure progress and am shocked by what I see: the current is sweeping me to the right, but I am not moving forward. It is sobering to swim at full pelt and apparently not go anywhere.

Luckily, the appearance is not entirely correct. Slowly but surely, land becomes closer and I reach the small island in the middle of the strait where the pink caps are already waiting. From there we swim on to Paros, battling the current until we clamber through the rocks onto the sandy shore.

We are all a bit stunned. This isn’t the Greek island-hopping adventure we’d expected. But when we learn that even the ferries have been cancelled, our morale is boosted. Nothing else will prove as demanding as that first short crossing.

The following morning, we move on to Koufonisi, where we join up with Akis, our smooth Spartan yachtsman, who will accompany us on our swims from now on. The afternoon is spent watching dolphins from the deck and doing a coastal swim beside a remote island. The colours are stunning. One minute the water is pale green, the next it’s a wall of deep blue, as the bottom falls into a steep canyon, and you almost want to stop swimming, sensing that you’re about to fall off a precipice.

Next morning, there is the dreaded stroke analysis in the hotel pool. We each swim two widths and one length of the pool while Melissa, our Australian guide, films us. Then, after breakfast, we watch ourselves. All amphibian life is there, in varying states of elegance. I look as if I’m on speed, all flailing arms and swivelling head. “Slow down your stroke, stop crossing over your hands, breathe less and follow through properly with your arms,” Melissa advises. And it works. Against my expectations, the improvement she brings to my technique makes this one of the highlights of the week.

Then we cross to Schinoussa, where we spend two blissful days. It is the most isolated and lovely of the islands, with a winter population of only 80 and a wonderful restaurant. By now, the ambience is relaxed and less competitive, and our misgivings about the high-fibre content of our fellow guests have been banished. We’re a mix of ages, abilities and eccentricities, and the holiday is the richer for it. THURSDAY, our last day of swimming, is the big one: Paros to Naxos, a 5.5km swim through one of the busiest shipping lanes in the Cyclades. “Yellow caps, you’ve got 15 minutes before you start,” Kate, one of the guides, shouts. The adrenaline is pumping. It will be a huge challenge not just swimming the distance, but dodging the huge ferries that thunder across. But then anticlimax. Akis feels the swell is too strong. Instead, we will do a 4km coastal swim around Paros, via the wreck of a German ship torpedoed in the war.

I’m gutted – but, 20 minutes later, the disappointment lifts. It is the most beautiful swim, through sun-streaked water, exploring dozens of little inlets with constantly changing underwater topography – and, because we are not going head to head with the ferries, we can take it as fast or as slow as we like. I linger in a cave while Jeanette flies off in tandem with one of the pink caps.

The last night is spent on Naxos, where the four least abstemious members of the party hold a beer-fuelled postmortem. We’re disappointed we didn’t do the Paros-Naxos swim. And, yes, not everything was as we expected. Contrary to expectation, it had not been a case of swimming from point to point: there was a lot of sailing, which most of us loved, but which had not been ideal for the guy who got seasick. But these are minor quibbles. Swimtrek has invented a great concept, one that binds you to your surroundings like no other trip.

The following morning, we climb the 1,000-metre Mount Zeus, and there they are below us: through the heat haze we can make out every island we swam to and from, spread out like dusty lizards asleep on a huge blue tablecloth. It looks rather impressive. Many people have travelled around the Cyclades, but there’s a certain satisfaction in knowing you’ve done it under your own steam.

Tom de Castella travelled as a guest of Swimtrek, Olympic Airlines and the Greek National Tourism Organisation

Travel details: Swimtrek (020 8696 6220, www.swimtrek.com) has a week’s swimming tour in the Cyclades (May-October) from £675pp, including half-board accommodation and sailing – where necessary – between the islands, but not flights or the ferry from Athens. Swimmers must be able to cover 3km-4km comfortably, although fins, floats and rescue boats are on hand if needed. (See box, left.) Fly to Paros with Olympic (0870 606 0460, www.olympicairlines. com), from Heathrow or Manchester, via Athens; or to Athens with Olympic, British Airways (0870 850 9850, www.ba.com) or EasyJet (www.easyjet.com). A return ferry from Piraeus (Athens) to Paros starts at £37pp; www.greekferries.gr.

How it works

Do I need to train?

Yes. Swimtrek provides a three-month training programme on its website (www.swimtrek.com). The idea is to build up consistently over time – it’s about stamina, not speed.

What if I get tired?

Once in the water, you’re never further than 20-30 metres from the safety yacht. The staff operate a standard lifeguard “5-20” scanning system – they can scan the entire group in five seconds and rescue them in 20.

What do I eat?

There’s no special diet, but it’s recommended that you tuck in. You’ll need plenty of food as insulation, as the water can get cold. Alcohol is banned during the day, as it lowers the core body temperature, but a glass or two of wine with dinner is certainly allowed.

Where do I stay?

There are three hotels on the various islands: Hotel Mantalena, on Antiparos; Hotel Iliovasilema, on Schinoussa; and Hotel Koufonisia, on Koufonisi. You’ll end up at Hotel Grotta, on the main island of Naxos.

SAMPLE ITINERARY

Day one: meet in Antiparos for a training swim and pretrip briefing.

Day two: swim from Antiparos to Paros.

Day three: the big swim to Naxos.

Day four: swim to Kato Koufonisi. After lunch, swim to Ano Koufonisi.

Day five: swim to Schinoussa.

Day six: swim to Iraklia. Boat to Naxos.

Day seven: Climb Mount Zeus to see where you’ve swum during the week. Feel smug.

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