Eight years ago, Capri Djatiasmoro watched helplessly as four Jet Skis ran over her friend while he was swimming at Brighton Beach. “They stopped to see if he was alive, but then they kept going,” she said. “He ended up at Coney Island Hospital getting 10 staples in his head.”

It had to be illegal for the Jet Skis to come so close to a beach where people were swimming, Ms. Djatiasmoro thought. And it was. New York State law prohibits personal watercraft from coming within 500 feet of a designated swimming area. Decades ago, a line of buoys ran parallel to the shore, marking the boundary between swimmers and boaters. But they had long since disappeared from the shoreline around New York City.

“Some boat and Jet Ski drivers don’t even know there is this law,” said Ms. Djatiasmoro, 72, a retired advertising executive and avid open-water swimmer who lives in Brooklyn. “Some know, but in the open water it’s hard to tell exactly where that mark is.” Jet Skis regularly come very close to the borough’s beaches, she said, especially in the late afternoon, after lifeguards have left for the day.

Ms. Djatiasmoro swims every day, and she organizes events where hundreds of New Yorkers swim long distances, such as from the Verrazzano-Narrows Bridge to Sandy Hook, N.J., or from Coney Island to Manhattan Beach. “I love the ocean, I love swimming, and I want to keep people safe while enjoying these things,” she said. She vowed to do whatever she could to protect swimmers. That meant finding out why the visible 500-foot boundary had vanished — and how to re-establish it.

That turned out to be more complicated than she expected.

First, she said, she met with Martin Maher, Brooklyn Parks Commissioner for the New York City Department of Parks and Recreation. That department, along with the New York Police Department and the U.S. Coast Guard, is responsible for the city’s waterways. The Parks Department’s position, she was told, was that replacing the buoys could lure swimmers farther out than they should go.

“Buoys can be useful for indicating the distance to the shore,” Chris Clark, a department spokesman, wrote in an email to The New York Times. “However, historically, we have observed that some swimmers view the buoys as goals to reach, causing them to swim out to unsafe distances.” For that reason, he said, “we have not used buoys in this area for this purpose since the 1990s.”

Ms. Djatiasmoro said she had also met with representatives from the Police Department and the Coast Guard. “Both said it’s not in their jurisdiction to place the markers — that it is up to Parks,” she said.

Other municipalities in the state have various approaches to enforcing swim zones at their beaches, and not all include buoys.

The town of East Hampton, near the tip of Long Island, has bay beaches on the north shore and ocean beaches to the south. There are buoys marking the 500-foot line on the bay side, said Timothy B. Treadwell, senior harbormaster with the town’s marine patrol. In the ocean, buoys “would be very hard to set and maintain, because of the movement of the water and because of the surf and the swell,” he said. “Even when we do it for lifeguard tournaments or open swim events, it is hard to get them to stay.”

At Atlantic Beach on the South Shore of Long Island, near the border with Queens, lifeguards are trained to alert watercraft if they come within 500 feet of shore, and to warn swimmers who venture out too far, said Nat Etrog, chairman of the parks and beaches commission for the Village of Atlantic Beach. This system is working so well, Mr. Etrog said, that he sees no need for buoys. But he said he understood why they might be helpful at Coney Island and Brighton Beach.

“It is very crowded there, and there are many more Jet Skis,” he said. “Their problem is much more significant.”

Ms. Djatiasmoro said she understood the possible downsides of buoys. “I kind of get it,” she said. “A kid or someone who isn’t that good at swimming might try and go out to the buoy, and then they might not be able to get back.” But she believes that swimmers face greater danger from watercraft.

In 2017, she started a petition, which garnered more than 300 signatures, calling on state and federal legislators to pass a new law requiring the Parks Department to install buoys 500 feet from shore at Coney Island and Brighton Beach, and to fine violators.

The effort found a sympathetic lawmaker in Pamela Harris, who at the time represented Coney Island and other southern Brooklyn neighborhoods in the State Assembly. She introduced a buoys bill that passed in the Assembly, but it died in the Senate. Shortly after that, Ms. Harris was indicted on fraud and corruption charges; she resigned in April 2018.

It was only after visiting friends in Fort Lauderdale, Fla., where Ms. Djatiasmoro learned from a local lifeguard that the swimming community funds and places its own buoys, that she got the idea to take matters into her own hands. Back in New York, a friend suggested she apply for a noncommercial permit to put out lobster traps.

“I called the Department of Environmental Conservation, and I said I wanted to put the traps out 500 feet and that I wanted to use buoy markers,” she said. “They said OK, so that is what I did.” She got her first permit in 2020 and has renewed it every year.

Jozef Koppelman, 65, a woodworker and diver who lives in Brooklyn and met Ms. Djatiasmoro through a swim event, took charge of making, installing and maintaining the buoys (which are not actually attached to lobster traps).

At the beginning, he made floating markers out of plastic bottles. “I think it was my Chinese mother-in-law’s cooking-oil gallon container or something like that,” he said. “I bought one of those corkscrew devices that you screw into the soil to leash your dog.” He had a friend swim out from Brighton Beach with a 500-foot rope, and then he dived to the bottom using scuba gear — the water is 12 to 15 feet deep at that point — and screwed the line into the sand.

Mr. Koppelman had been paying for the materials out of pocket, but this year Ms. Djatiasmoro organized a GoFundMe campaign just before Memorial Day to raise money for the season. “We asked for $1,000 and got to $1,700 within two to three weeks,” she said. Supporters of the initiative held a party at Mr. Koppelman’s woodworking shop in Gowanus to assemble the buoys.

“The design has gotten more elaborate each year,” Mr. Koppelman said. He now installs foam buoys that he weighs down with cinder blocks that weigh 40 to 50 pounds each.

He dives on the weekends to clean the cinder blocks. “It’s amazing how quickly they attract marine life,” he said. “You have crabs and small fish and the line gets full of seaweed.” One of the hardest parts is avoiding tiny marine arthropods that instinctively attach to surfaces, including his skin.

The buoys also tend to disappear: Of the nine Mr. Koppelman has installed at various points along the shore in Coney Island and Brighton Beach, seven remain, though he tries to stay on top of replacing them. “I think that some of the Jet Skiers must resent them,” he said, adding that people had also sometimes removed the flags attached to the buoys.

One of the people who donated time and money to the project is Jeremy Whelchel, 40, a software engineer and open-water swimmer who lives in Brooklyn.

He said he had experienced firsthand boats and Jet Skis getting too close. “When you are swimming you can hear them under water, and sometimes they get really close to you, and you don’t know if you are visible to them,” he said.

He said he believes the buoys have helped eased tension between swimmers and Jet Skiers, who have a reputation in his circles for being reckless and macho.

“From a swimmer’s perspective, we always think these watercraft are too close,” he said, “but there have been many conversations this summer where we thought they were too close, but they were on the outside of the buoy,” he said. “So it’s like, ‘OK, we can’t get mad at them.’”

Despite the vandalism, no organized opposition has formed and no city agency has tried to stop the project, so the D.I.Y. buoys seem to be here for the time being.

Matthew Fermin, the owner of Rockaway Jet Ski, a ski rental and tour company based at Rockaway Beach, said everyone should be in favor of them. “It’s not a big deal to put them up, and it’s an easy way to keep people at a distance, so why not?” he said.

“The Statue of Liberty has buoys around it that say ‘Don’t go past this mark,’ so why can’t the beaches?” he added. “There isn’t even anybody swimming near the Statue of Liberty. Why should there be a sign there and not in a place where there is human life?”