Ira Gershenhorn was in a full sweat by the time he bicycled up to his swimming spot on a recent sultry afternoon.

He stripped down to his bathing suit, put on a yellow bathing cap and slid into the Hudson River, off Manhattan’s rocky western shoreline, around 104th Street.

“Ah, feels great,” said Mr. Gershenhorn, 66, as if he had just dived into a pristine swimming hole and not a sometimes unsanitary stretch of the Hudson plied by tugs, tankers and barges.

Users of the nearby bike path did double-takes to see a bather in these waters long considered dirty and unsuitable for swimming — a perception that Mr. Gershenhorn, a clean-water advocate and a regular swimmer in the Hudson, calls outdated.