In 1996 the British writer Roger Deakin decided to swim his way around the British Isles, starting the journey from the murky Elizabethan moat on his farm. His chilly adventure had transformative effects. “When you enter the water, something like metamorphosis happens,” he wrote in Waterlog, the 1999 book chronicling his odyssey. In the 21 years since, Waterlog has become something of a bible to proponents of “wild swimming”: an adrenaline-raising trend nudged into popularity by its publication.

But why call it “wild”? Perhaps it’s because we’ve domesticated the act and become accustomed to man-made pools. In swimming pools, you can see what lies beneath. But in a river, it’s all left to the imagination, for better or worse. In our sanitized lives, as we contemplate wa...