Four years ago, three guys had an idea: that it might be nice to swim in the East River. “It came about pretty simply,” one of them, Dong-Ping Wong, recalled recently, on one of those humid, ambition-melting summer mornings. “Just sitting and sweating near the river, and realizing, after years of living here, that it’d be sort of amazing if you could jump in, and kind of ridiculous that you couldn’t.” The N.Y.P.D. frowns upon people jumping into the city’s waterways, out of practical concerns about the strength of the currents and the difficulty of exiting—no ladders, no ramps. Also, consider the quality of the water itself, which seldom conjures up images of the Caribbean. The guys decided that they should build a floating pool, one that would neutralize the current and filter out contaminants. No chlorine: that would be cheating. Just the river, with lifeguards and without the urban jetsam. They called it +Pool, owing to the intended design scheme, which features four adjoining rectangles in the shape of a plus sign.

Wong, who is thirty-four, was standing in a boathouse at Pier 40, on the western edge of Houston Street, with his fellow wannabe swimmers Archie Lee Coates IV and Jeffrey Franklin. Using Kickstarter, they have raised more than three hundred thousand dollars for the project, mostly by selling tiles for the eventual pool, on which donors can request that their names, or short messages, be printed. One such tile, for instance, says, “Push here for hot dog.” Another, from a man in Chile, says, “Close Guantánamo.” Wong added, “One of ’em just says, ‘Toilet,’ ” and giggled.

“ ‘Toilet’ is a good one,” Coates said. He meant this in light of the challenge that he and his partners face as river-pool engineers. Before they can approach the city for permission to build, they need to figure out how to filter the water. Some days, the water is clear enough that you can strain it with a Brita filter. (This is not recommended if you hope to use the Brita again.) But when it rains hard—a quarter of an inch in an hour might do the trick—some four hundred sewer pipes around the city begin flushing untreated waste, using the rivers as municipal lavatories. “We’re kind of pussies,” Wong said, noting that none of them had yet taken the plunge, either in the East River or in the Hudson, where they are now focussing their studies.

The +Pool men, who are designer-architect types, were testing potential filters. In a corner of the boathouse stood a tall Fluidyne system, a machine for pumping water up through a series of coat-liner-like fabrics, one of which they’d nicknamed Cookie Monster, because of its fuzzy electric-blue sheen. Pipes connect the machine to a floating dock outside, where more filtering experiments take place. Square holes have been cut into the dock, like sample pools, framed by three layers of screens, each separated by inflatable bike-tire tubing. In one of the sample pools, the fabrics have been fashioned in a corrugated pattern, yielding better results—more surface area for the fecal effluvia to cling to. A fourth experimenter dangled a black-and-white Secchi disk into another pool, to gauge the water’s clarity. The disk disappeared from view at a depth of fifty inches, an improvement of two feet over the murkier chop splashing around beyond the dock’s edges.

Later this year, they plan to publish a report on what they’ve learned so far. (Realistically, they don’t imagine Opening Splash taking place until 2016.) For the statistically inclined, the figure to watch is the number of colony-forming units (of bacteria) per hundred millilitres. Beach advisories are often set at thirty-five. Back in the boathouse, an environmental engineer named Greg Grzybowski retrieved water samples from an oven where they’d been baking for twenty-four hours, and held them under fluorescent light. A few samples glowed, indicating the presence of bacteria. Grzybowski performed some quick calculations. The raw river water scored a hundred and thirty-five: yuck. The cleanest +Pool water registered thirty—passable, but barely.

Grzybowski noted a brackish tank nearby, where a couple of pinkie-finger-size seahorses that had been fished out of the harbor were now bouncing around. “I would swim in a river that a seahorse swims in,” he said. “Maybe that’s just me.” A somewhat experienced Hudson River swimmer (north of Manhattan, that is) contemplated the conditions, and the likelihood of getting busted, and returned unhappily to an office in midtown—where, a few hours later, his phone vibrated with an emergency flash-flood warning. The skies opened. Grzybowski sampled again, and the C.F.U. count in the river exceeded 24,196, the highest possible measurement. “Pretty much raw sewage,” he reported. The would-be swimmer felt a wave of compassion for the seahorses. ♦