For Ned Merrill, the hero of John Cheever’s 1964 short story, The Swimmer, immersion in water was “less a pleasure . . . than a resumption of a natural condition”. Cheever’s protagonist finds himself at a cocktail party in an affluent suburb of Westchester County, New York, and decides to swim home via a series of pools.

But as his quest progresses, Merrill’s psychological deterioration becomes evident. He emerges from each pool yet more sharply delineated as a contemporary Narcissus, amnesiac of a series of disasters, able only to return by water to his childlike self.

Merrill therefore makes a curious inspiration for Roger Deakin’s swimmer’s travelogue through Britain, Waterlog. Merrill’s ability “to see, with a cartographer’s eye, that string of swimming pools, that quasi-subterranean stream that curved across the country” lingered with Deakin, who decided to set out on a swim through Britain.

But Deakin’s year-long, mostly watery journey made for an uplifting book, less Merrill and Narcissus, more hymn to the metamorphosis wrought by entering the sea, rivers and lakes and — when he reached Penzance — Britain’s largest surviving seawater lido, the Jubilee Pool.

Here, within the flowing Art Deco lines of a pool opened in 1935, Deakin met Madeleine, a local painter who swam daily. In the huge, triangular, open-air lido, Madeleine averred that “swimming is better than sex”. Deakin is unsurprised by her devotion to swimming in so sensual a place as the Jubilee Pool.

Freshly painted in blue and white, having been saved from a local council whim to redevelop it as a modern “fun pool”, Deakin’s surrounding seem exuberant, extravagant and romantic.

The Jubilee Pool was designed by Penzance’s borough engineer, Captain Frank Latham, at a time when Britain was lido crazy. In 1935 lidos also opened at Ilkley, Norwich, Peterborough, Saltdean and Aylesbury.

Deakin adds that the country’s fervour for outdoor swimming owed much to Weimar Germany, for there lidos were the symbolic heart of the new cult of the body, one which embraced sunshine and sunbathing in a way that would surely be frowned upon today.

Perhaps a concern that its citizens were getting too much sun animated the council when it proposed transforming Jubilee Pool into a new, “fun” version of itself, but its lofty aims were stymied by John Clarke, a retired assistant county architect for Cornwall, who got the pool listed as a Grade II building.

I arrived at the Jubilee Pool in June. It was a sultry day, and the multi-coloured flags heralding Penzance’s annual Golowan Festival fluttered in a balmy breeze. There were just two swimmers in the water.

I watched them and gazed across Mount’s Bay to St Michael’s Mount and then, to the west, the ancient fishing village of Newlyn. Before long, going for a swim was irresistible. I slid into the warm sea water and swam uninterrupted lengths.

Refreshed, I made my way to the café. An arty-looking woman was drinking a ginger beer. Could she be Madeleine, still visiting the pool a decade or so after her conversation with Deakin?

The lady’s name was Demelza. She wasn’t an artist but came close, being the owner of a Padstow gallery called Beyond the Sea. Demelza swims at the Jubilee Pool with the same fidelity as Madeleine.

I didn’t ask whether she, too, believed that swimming was better than sex, but I did learn that, for her, the allure of the Jubilee Pool is no less intense. As she put it: “It’s the most magical place in the world.”