"Fifty years ago, this was one of the most polluted rivers in the whole world," said Pugh, 53, who was appointed the first UN patron of the oceans a decade ago.

"We need to have clean, healthy rivers," he told reporters after completing the unassisted trip from the Hudson's mountain source all the way down to New York City.

He said that in New York's industrial past, the river would sometimes change color from day to day, depending on what dyes and other pollutants were dumped or ran off.

But after decades of action to clean up pollution, Pugh was able to safely swim down the river, a month-long feat he said will hopefully inspire others.

"They're going to be inspired by what happened here and say to themselves: 'If they can do that in the Hudson, surely we can do it in our river and our river can also be saved,'" he said.

Pugh has previously undertaken high-profile swims in Antarctica, the North Pole and the Red Sea to advocate for rivers clean enough to swim and fish in safely.

His Hudson River feat comes as leaders from the around the globe are set to descend on New York to attend the annual UN General Assembly, during which a historic High Seas Treaty is to be formally signed.