Extreme swimming: Who needs ferries to island-hop in Greece when you can jump in and swim between them instead?

The throb in my shoulder has deepened. The lacerations on my hands are so minute they are nigh-on invisible, but sting with every movement. My eye has stopped weeping but my tongue still feels huge, and I have been barely able to talk for two days. My right forearm is red-raw; and the second toe on my right foot is a distressing shade of inky blue-black. I've never felt so happy in my life.

It is day five of the week-long SwimTrek tour I have joined in the Greek Cyclades, a group of small islands (some with populations of hundreds, others with no more than a stray goat or two) and the aches and pains have been building. (The shoulder from, well, a lot of swimming; the scratches courtesy of tiny sea creatures; puffy eye and bloated tongue thanks respectively to goggle misplacement and opening my mouth too much underwater; my forearm from dragging it too closely up my side; and the toe from stubbing it on a reef – goodness knows the water was clear enough; I just failed to see it because of my puffy eye.)

I am swimming with Owen, a lieutenant commander in the Irish Navy and erstwhile captain of the Rattle & Hum. I believe he is about a foot-and-a-half to my right. I can't say for sure, because the waves have picked up as we make our way out to a craggy rock no more than five metres in diameter, rising high out of the sea just east of Kato Koufonisi. I haven't seen Owen for a minute, but I'm not about to slow down. Though it is good to have company in the middle of the sea, I'm sure he's not about to give up, and neither am I.

Our (or at least my) macho idiocy lasts another 30 seconds, as the smell of diesel fills the air. Our tour guide, Andy, draws up alongside us in his high-powered dinghy to tell us to head inland as it's too choppy out here – something Aki, our Greek guide in the main boat ahead seemed to be signalling to us a couple of minutes earlier; our female counterparts, who had been 50 or so yards behind us, had bent their path landwards a few minutes before Andy stopped us, and are now waiting at the cove that was our ultimate destination. Still, who wants to be sensible when you have a whole ocean to play in?

The ladies are no mean swimmers themselves – and clearly a lot more cognisant of sensible routes. On the first night, when we had introduced ourselves, I'd felt rather intimidated. Alongside Captain Owen, a regular triathlete, were Mary, his wife, an electrical engineer in the same force, and fellow triathlete; Karen, a San Franciscan who regularly "escapes" from Alcatraz, swimming to the bay; and Marcia, who works for the Australian government, is as graceful in the water as Flipper and has been a swim-stroke instructor for many a year. She is also the mother of our guide, Andy, who himself once achieved a qualifying time for the Commonwealth Games. I myself have never swum properly before in open water. No pressure, then.

I needn't have worried. An intensive course in open-water technique had me twisting my body 180 degrees, keeping my stroke shallow and my elbows high, dragging my arms up my sides and gliding within hours. The biggest challenge that remained was to remember to breathe.

But learning, and improving on, technique is a great joy of open-water swimming (SwimTrek caters to anyone aged from 16 to those in their sixties), and it is not unusual to find yourself thinking of nothing else for hours on end while traversing the deep blue (not to mention talking about it over dinner).

Over the course of five days, we swim a total of 18km (followed on the final day by a hike up Mount Naxos). Travelling along the coastlines of Schinoussa, Koufonisi and Iraklia is spectacular (no more so than when venturing into a cave to find three seal pups – and their mother, who promptly dived under us, rocketing away). But it is the island crossings, swimming 3km-plus at a time, that gives the greatest sense of achievement. The only drawback: it is addictive. Next stop: the Hellespont, crossing from Asia to Europe with SwimTrek to mark the bicentenary of Lord Byron's achievement in 1810. Really must remember to keep my mouth closed next time...

For more on SwimTrek, including the Hellespont trip, visit swimtrek.com. EasyJet (easyjet.com) flies to Athens from Gatwick daily, with fares from £61.98 return, including taxes