One of the more romantic versions of NYC in the summer involves taking a nice little walk inside one of the city's fountains. Of course, this isn't Fellini's La Dolce Vita, it's real life, and in reality that fountain is just a glorified puddle filled with city filth. Imagine all of the things that live on a subway pole, extracted and liquified and poured into a open-air bath in the middle of the city... Still, when it's this goddamn hot outside, Washington Square Park's fountain calls to you like a siren. So, is it legal to just walk into any fountain in NYC?
Hell yeah it is.
It is not against park rules to go into display fountains, though the Parks Department may discourage it as they are not intended for swimming, and as such the water is not treated in the way a swimming pool is. But, you can still hop in, as long as you are not using it to bathe—it is against park rules to use fountains for personal hygiene or for washing clothes, belongings, or pets (there are water fountains for that!).
Washington Square Park's fountain is probably your best bet for cooling off in this way; earlier this year the NY Times spoke with George Vellonakis, executive director of the Washington Square Park Conservancy, and reported that "the fountain holds almost 32,000 gallons of water between its two tanks... That water is constantly moving, recirculating between a filtration system (which keeps the fountain clean) and display pumps (with jets that can shoot the water 45 feet into the air)."
All of the city's fountains (50 in total) get their water system cleaned before being turned back on for the summer. We're also told that "they are cleaned of debris as needed by maintenance crews when they service our parks." But that ain't stopping the rat-pigeon pool parties that definitely happen when we aren't looking.
N.B.: You probably shouldn't bother trying to watch an outdoor movie in Bryant Park while in the fountain. Bryant Park is managed by Bryant Park Corporation, and they prohibit humans in the fountain, which is behind the screen anyway. Just bring your own kiddie pool and drag it out onto the lawn before the movie starts.