Despite the fact that the city is mostly surrounded by water, you won't find anyone swimming off the shores of New York. That'd be nasty. Nearby rivers suffer from ongoing pollution issues caused by PCB contamination, accidental and non-accidental sewage discharges, urban runoff, heavy metals, furans, dioxin, pesticides, and polycyclic aromatic hydrocarbons. You know, shit that makes mutant fish and whatnot.
I'm not saying people should ever swim in the toxic rivers, but wouldn't it be nice? A few dudes in Brooklyn think it's a cool idea, so they're planning to build a plus sign-shaped pool, appropriately named Plus Pool, that will float in New York's surrounding waters. What's nuts is the thing will actually be filled with river water that it'll have cleaned because the pool is a huge filter, too.