"Just don't think about it," chuckled Roman Karkachau as he emerged from Tooting Bec Lido yesterday, "it feels like a bath."
The barrel-chested Russian – to my eyes half man, half sea lion – was seeking to offer some reassurance before it was my turn to take to the south London waters. But while Mr Karkachau is more used to breaking the ice on Lake Ozerki near his home in St Petersburg before his daily constitutional, I prefer a few leisurely lengths at the local (heated) health club pool.
We are gathered in Tooting ahead of this weekend's winter swimming championship when some 670 swimmers will battle it out in a series of races and endurance events.
It is the first time that the championships have been held outside Finland and despite fears that the mild winter here would render the historic lido's water too warm for international competition, dozens of competitors were reporting conditions to be suitably refreshing during a pre-match acclimatisation session in front of the world's television cameras.
With the pool thermometer at a distinctly nippy 6C (43F), I was warned that taking the plunge would indeed "sting a bit". Actually it stung a lot. Slowly descending the ladder, the first sensation is one of sheer incredulity that it is possible to feel quite this cold. As the stomach, chest and major organs are immersed, the sense of disbelief gives way to breathlessness and a soaring heart rate. After two widths it was all I could do to drag myself gasping out of the water and drop to the ground to drink in the un-seasonal sunshine. But then the tingle begins.
The fact that 40 or so members of the South London Swimming Club come here each day throughout the winter is testament to the power of the tingle. As the lacerating cold subsides, the entire body is engulfed in the warmest of glows. Five minutes later you are positively zinging.
Margi Sullivan, 51, who has been swimming here for nearly 20 years, admits to something of a tingle habit. "You definitely get addicted to cold water swimming," she said. "That is absolutely definitely true. I swim here all the time and it makes you feel fantastic."
There are many claims made for the health benefits of cold-water swimming. Supposedly it can cure everything from depression to drooping breasts. But casting an eye around the celebrated bathing lake, it is clear that all the swimmers assembled look considerably younger than their age.
Tarja Krohns, a 47-year-old maths teacher from Oulu in northern Finland, is one of 300 Finns taking part in this weekend's festivities. Every morning she takes a dip in her local Oulu river where snow lays thick on the ground and the air temperature drops to -20C. "It feels so good," she said. "Some people used to think I was crazy but not any more. I have never been sick in the past 10 years. It is very different here – we are getting brown in this sunshine."
According to David Dunham, a retired teacher who combines his duties as official temperature-taker with a couple of daily widths, says we have never had it so mild. "It just seems to get warmer each year," he said. "We have only had ice on one day and then it was soon melted. We usually get four or five days but in the old times you used to have to regularly break the ice to get in."
In fact, excruciatingly cold though it may have felt, this weekend's conditions are unusually benign. "Everybody can go in today," said Jonathan Buckley, a regular Lido-goer . "It is when it is grey and the north-east wind is whipping across that it feels really hard."
But then, there will always be the tingle to look forward to.