Wearing a fluorescent orange life preserver and rubber gloves that reached his biceps, Prince William waded — ever so carefully — into New York’s East River. A minor slip might have been embarrassing. A splash? Nothing short of an international incident.

It had been raining since early morning. Damp onlookers watched the prince toss a few juvenile oysters in a bucket, then wade through waist-deep water until he reached the shore.

Relief was palpable among staff of Billion Oyster Project, a nonprofit aiming to restore oyster reefs to New York’s waterways. The prince’s visit with the organization on Monday had been years in the making: It was postponed last September after the death of his grandmother, Queen Elizabeth II.

This time around, the staff’s preparations included ordering new waders, the type of waterproof overalls worn by fly fishermen. “We were like, what size waders does the prince wear?” Jessi Olsen, the corporate partnerships manager for Billion Oyster Project, said.

“He looked like a natural,” Agata Poniatowski, the organization’s public outreach manager, said. “I believe he’s been in waders before.”

Prince William was visiting New York for two days that coincided with Climate Week, a summit on climate action that takes place alongside the United Nations General Assembly. He met with the U.N. secretary general, António Guterres, Monday night and on Tuesday, Prince William will announce finalists for the Earthshot Prize, awarded by the climate-focused charity he founded in 2020.

His first stop, though, was a pile of shells on Governors Island. The prince arrived on a silver T-boat, a 28-foot long passenger vessel. He was accompanied by security personnel with life vests layered over their navy blue suits.

Around 3:30 p.m., he walked into a tennis court-size enclosure to the southeast of the island, where he was surrounded by mounds of oyster shells up to eight feet high.

The shells had been donated by restaurants including Raoul’s and La Marchande, their contents already slurped out by diners. The shells then arrived at Governors Island to be cleaned (and separated from accidental detritus, like hot sauce packets).

Founded in 2014, the project aims to restore one billion live oysters to a harbor where the mollusks once thrived. Some shells are placed directly into New York Harbor to be taken up by oyster larvae, and others are fostered in oyster nurseries. The oysters are intended not to be eaten but to improve the harbor’s biodiversity and to protect the city against flooding.

From a six-foot pile, the prince plucked a shell and rubbed it between his fingers, as if contemplating its potential. According to Pete Malinowski, the executive director of the Billion Oyster Project, it has 130 million oysters down and 870 million to go.

By 4 p.m. the prince was whisked on a blue-and-white golf cart to Pier 101, where the same boat was waiting to carry him to Brooklyn Bridge Park. He maneuvered down a narrow, slippery gangway to the dock. A Coast Guard boat churned through the waves ahead of him — two more bobbed nearby.

Lasting just over an hour, the prince’s visit was hushed, orderly and tightly choreographed — noticeably different from some other displays of climate activism that took place in the city this month.

During the U.S. Open, protesters, one of whom glued his bare feet to the cement, interrupted the semifinal between Coco Gauff and Karolina Muchova. At New York Fashion Week, protesters walked onto the runway at the Coach show. And this weekend, at least 16 climate activists were arrested during a protest at the Museum of Modern Art.

Prince William, who was kept carefully beyond earshot of reporters, could not be asked for comment on the different approaches.

Mr. Malinowski, the Billion Oyster Project director, said he was not aware of the MoMA protest. “I think everyone has to do their part, however they can do it,” he said.

He emphasized that the project teaches young people how to make hands-on improvements to the health of the planet. Staff members teach students at the New York Harbor School, a public high school on Governors Island, about aquaculture, ocean engineering and marine policy.

“There’s not a lot of ways for young people to have a positive impact on the planet,” Mr. Malinowski said. “Most of what we’re taught is how to minimize our negative impact.”

Emma Brech, 22, a student who lives in Long Island, had traveled to Governors Island before dawn, hoping for a glimpse of Prince William. The prince’s attention to the climate crisis “gives me more hope for the future,” she said.

As it rained for hours, with no sign of the prince, Ms. Brech huddled under an umbrella printed with the Union Jack. Her patience was rewarded when Prince William waved to her briefly on his way off the island.

“Selfie?” she called out. No response.