Summer is the dangerous season: a warm lake, a long swim and cold I did not feel
In winter, cold water announces itself and every decision is made for you by the clock. In August the lake read 21 °C at the surface when I dipped the thermometer at the jetty, the air was 26, and I did what I never do in January: I set off without a plan for how far or how long, feeling warm and strong, and swam along the reed bed to the far buoy and back, which is about 1,200 metres. The longest swim of my year by a distance.
About two-thirds of the way out, maybe 800 metres in, my legs began to feel heavy and slow in a way the rest of me did not. The explanation was under me. Lakes in summer stratify: the top metre or so is warm and the water below it is still spring-cold, often ten degrees colder, with a sharp boundary between them. Swimming front crawl, my arms and chest were in the warm layer and my legs were kicking through the top of the cold one. When I stopped to tread water at the buoy and my feet dropped a metre, the cold was startling, like stepping into a different lake.
The swim back was fine but slower, and I had a mild calf cramp at about 1,000 metres that I kicked out of. Getting out, I was not cold in any obvious way. Twenty minutes later, dressed and drinking tea, I was shivering steadily and my hands would not do the flask lid, which told me I had been losing heat for most of the swim without the warning signals I rely on in winter. The surface temperature had lied to me, or rather I had asked it the wrong question.
What changed afterwards. The thermometer now goes down a metre on its string at the start of a summer swim, not just at the surface, and the lower reading is the one I plan from. Summer swims get a distance and a time decided before I get in, the same as winter swims do. The tow float was with me, which I was grateful for at the buoy; the swimming partner was not, because it was a warm day and it felt unnecessary, and that is exactly the reasoning I would not accept from myself in February.
Most people who get into difficulty in open water do so in summer, and I used to think that was because summer is when the inexperienced get in. Some of it is also because summer is when the experienced stop paying attention.