Like most people who have heard anything about the Gowanus Canal — of its murky sludge, oil slicks, raw sewage, carcinogenic chemicals and dying dolphins — Christopher Swain strongly advises against swimming in it.

Unlike most, however, Mr. Swain can now speak from experience. An environmental activist who says he was the first to swim the lengths of the Charles, Columbia, Hudson, Mohawk and Mystic Rivers, Mr. Swain, 47, spent nearly an hour on Wednesday afternoon performing what he called a “modified head-up grandma breaststroke” in the Hulk-green, foam-smeared waters of one of North America’s most polluted waterways.

“It’s just like swimming through a dirty diaper,” he told a crowd of reporters, residents of the nearby neighborhoods and incredulous passers-by who had gathered to see him off at the canal end of Degraw Street. Having some experience with dirty diapers, as well as with other, somewhat less filthy waterways, Mr. Swain had come prepared: His silver Nissan Pathfinder held a pair of black rubber fins, a yellow dry suit, a pair of black gloves, goggles and a green rubber swimming cap on which Mr. Swain had written, in black marker, “#HOPE.”

The full-body protective gear explained itself; the hashtag represented his reasons for immersing himself in the deadly cocktail of a canal, into which factories and Brooklyn’s sewage system have been dumping their waste since the late 19th century. Though the Environmental Protection Agency has designated the canal a Superfund site and plans to dredge up the toxic sludge at its bottom, Mr. Swain wants to see the day when the Gowanus is so clean that anyone can swim in it.