This being a G7 summit armed to the teeth with heavy security, he was not exactly alone.
French police frogmen watched from nearby power boats, while one officer on a paddleboard kept an eye on the slightly eccentric vision of a world leader crawling through the waves.
Johnson had been accompanied on his dip by the UK’s ambassador to Paris, but No.10 insiders confided to HuffPost UK that Ed Llewellyn had resisted the temptation to go all the way out to the rocks and back.
As a former chief of staff to David Cameron, Llewellyn is used to Eton-educated Tory premiers with a penchant for wild swimming.
Cameron took to the waters of Lough Erne during the G8 summit in Northern Ireland in 2014, saying: “It certainly wakes you up.”