Swimmers at the starting line in Brooklyn Bridge Park.

On Sunday morning, 400 or so swimmers dove off the banks of Brooklyn for the annual Brooklyn Bridge Swim, a sprint across the East River to a tiny, rocky beach on the Manhattan side. It’s a short swim, but it takes guts: most New Yorkers think of the East River as a stinky, polluted canal where the city pumps sewage and the mob dumps bodies. "You're swimming in the East River?" Jerry Seinfeld asks when Kramer decides to start doing laps outdoors. "The most heavily trafficked, overly contaminated waterway on the Eastern Seaboard?"

The truth is, the rivers in New York today are clean enough to swim in. The East River has an unattractive, greenish tint, and a few floating Doritos bags, sure. But on most days, the levels of bacteria meet federal safety guidelines, according to state and local officials. Even when the bacteria levels in the water are high, it’s unlikely that swimmers will get sick. If they do get sick, the severity will probably be more along the lines of eating bad Chinese food than setting off a cholera outbreak. In past open-water events in New York, heart attacks have been more prominent than pollution-induced disease.

Still, there is a deep stigma attached to swimming in the city’s waterways. Friends recoil when Gustavo Leal, an assistant swimming coach at Columbia University, mentions his open-water swimming. "They think it’s pretty nasty, it's dirty and whatever, that you’re swimming to a dead body or something," he said. "I’ve never heard of anybody getting sick. I think it's more of a myth than anything else."

There is a deep stigma attached to swimming in the city’s waterways

Morty Berger, the founder of NYC Swim, the group that stages open-water races in New York, refused to comment for this story. NYC Swim has "been burned before" by press, he told The Verge, and no media would be allowed at the event. (The media covered it anyway. UPDATE: Berger said he never claimed media were not allowed to cover the event, as it was in a public park; just that outside press were not allowed inside the swimmer's area.)

That’s probably because it’s impossible to talk about swimming in New York’s waterways without talking about poop. About 60 percent of New York’s sewage system still relies on "combined sewage overflows," or CSOs, pipes that dump sewage and stormwater directly into the rivers when it rains. More than 27 billion gallons of this sewage-stormwater mix drain into the city’s waterways every year, according to the conservation group Riverkeeper.

"New York City is full of shit. Real shit," opens the Vice mini-documentary about the city’s waterways. "Braving Possible Poo, Hundreds Swim In East River," read the Gothamist headline on a story about the 2011 Brooklyn Bridge Swim.

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The city has promised to invest $5.2 billion toward cleaning up the water over the next 10 years. The plan includes installing green roofs and permeable pavement to absorb rainwater before it enters the sewage system. The city is also adding capacity to its wastewater treatment plants so that rainwater can be treated before being dumped into the harbor.

Some advocates say the city’s efforts aren’t enough. "They have a plan to start to try to whittle away at the problem," said John Lipscomb, who manages the water quality testing program for Riverkeeper. "Riverkeeper doesn’t think it’s good enough. It's not soon enough."

While the average water quality may be much better, the bacteria counts in different spots along the rivers vary wildly. Sites such as Newtown Creek and the Gowanus Canal may have 10 to 100 times the federally recommended level of fecal bacteria, according to Riverkeeper’s testing. Bacterial counts are also much lower in the middle of the river, where the state’s numbers come from, than on the shores where people are actually swimming, Lipscomb said.

It's impossible to talk about swimming in New York’s waters without talking about poop

Between the damage from Hurricane Sandy and Mayor Mike Bloomberg’s $20 billion waterfront revitalization plan, there is more money focused on cleaning up New York City’s water than ever before. The water is also cleaner than it has been in more than a century, but it’s still not so long ago that New Yorkers joked about the "syringe tides" of medical waste washing up on the beaches.

Awareness of the issue is growing as well. The Sewage Pollution Right to Know Act, passed in the summer of last year, is a state law that will require municipalities to report exactly when sewage is discharged into the water.

On Sunday, the swimmers were more concerned about treading through the powerful current than taking unavoidable sips of brackish city water. The cleanliness of the water "is not a deterrent for me," said Jennifer Bolstad, who competed in the Brooklyn Bridge Swim swim for the third time. "People ask me, ‘How many times do you have to shower after doing that?’ The answer is three, before you stop smelling strangely metallic."

Swimming in the water around New York has made her braver, and she’s no longer afraid of water pollution (except for Newtown Creek and the Gowanus Canal, which have both been designated Superfund sites, meaning the EPA considers them top-priority hazardous waste sites).

There’s also a sense of obligation. "It's kind of a polemic in the city," she said. "If you want the water to be swimmable, someone should go and swim in it. We're an island city. We're surrounded by water. We should make a point of using that water."