City officials agree, calling the Hudson’s water quality the best it has been in a century. But health advisories are sometimes issued when heavy rainfalls overwhelm New York City’s sewage treatment capacity and send untreated wastewater into the river through outflows along the shoreline.

Still, parks department officials called it inadvisable to swim in the Hudson, which has strong currents, no lifeguards and few access points. Officials from the Health Department would not say if the water quality was safe enough for swimming.

Mr. Gershenhorn, a computer programmer and a good swimmer, is driven by convenience — he can be in the water within minutes of leaving his Upper West Side apartment building — and by the mission of convincing Manhattanites that the Hudson is swimmable.

“I’m just going in to show people it’s O.K. to go in the water — it’s a political reason,” he said, while breast-stroking in a light chop, with a reporter swimming alongside.

“It’s good the wind’s blowing north because the stink from the sewage treatment plant won’t affect us,” he said, referring to the North River Wastewater Treatment Plant, up river.

Between dips, he pulled trash strewn between the boulders, part of a shoreline regimen for Mr. Gershenhorn, who travels with a trash-picking pole on his bicycle for gathering garbage. He also carries materials for planting seedlings of native plant species along the river.

As a volunteer with the New York City Water Trail Association, a paddling group that runs weekly water quality testing of city waterways, Mr. Gershenhorn takes water samples along the Hudson and rides them to a testing site by bicycle.

This lets him monitor the water quality around his bathing spots. He avoids swimming after heavy rains, he said, but largely, water cleanliness is not a concern.

“I come home after swimming and I have to remember to take a shower because I don’t feel dirty,” said Mr. Gershenhorn, who has no problem opening his mouth or his eyes underwater.

Of course, the Hudson’s murky waters offer no real visibility — a good thing, Mr. Gershenhorn said, because “I don’t want to know what’s swimming in there with me.”

And the river’s pollution may not be all that bad for the various species of marine life, he said.

“They actually do better around it — it’s a crazy thing,” said Mr. Gershenson, who grew up on Long Island swimming in the ocean and bays. After moving into Manhattan, he would frequent beaches in Brooklyn and Queens.

“Then I thought, ‘Why can’t I just jump in the river?’” he recalled. “So I started doing that, and it was much easier. You don’t have to spend the whole day going back and forth to the beach.”

Mr. Gershenhorn is also a volunteer with the Billion Oyster Project, an effort to restore a sustainable oyster population in the city’s waterways, and he holds weekend sessions explaining local water issues to the public on the Baylander, a decommissioned Navy vessel docked at West Harlem Piers Park, at the end of 125th Street.

During cooler months, Mr. Gershenhorn is known to Riverside Park users as the Lord of the Rings, for his weekend practice of providing play equipment near the swinging rings at the Hudson Beach section of the park near 105th Street.

Mr. Gershenhorn has been a vocal supporter of river access along the shoreline of Manhattan, the only New York City borough without an official city beach.

There are some sandy spots along its shorelines that Mr. Gershenhorn said could be opened by the city as beaches, including a stretch of Hudson shoreline just below the George Washington Bridge. In 2016, Mr. Gershenhorn presented a proposal to make it a beach to the local community board, which held a hearing but took no action.

“Years ago, a lot of New Yorkers used to swim in the Hudson, and so did their parents, back when it was even filthier,” he said. “But now we have this obsession with cleanliness.”