Hold that cannonball!

The Gansevoort Hotel’s uber-cool rooftop pool belly-flopped in a Post test for water cleanliness, scoring last among six city swimming spots.

The five that outclassed it? The city’s public pools.

The hotel pool was the only one to test positive for coliform bacteria, which could indicate possible fecal contamination, and had the highest levels of “solids” — organic and mineral particles that shouldn’t be there.

There was also a high nitrate count and high sodium level — possibly indicating some guests aren’t just leaking celebrity gossip.

The Gansevoort — a swank Meatpacking District hotspot known for gorgeous women lounging in bikinis and boldfaced names in the registry — charges $700 for a room, or $300 a day to just use the pool, gym and spa.

“Pools do show bacteria from time to time, depending on how many people are in the pool and how many ‘accidents’ they have,” said biochemist Thomas Mullen, of Water Test America, which analyzed samples from the Gansevoort, and public pools in each borough.

The Gansevoort, which didn’t return a Post message seeking comment, “might have to up their chlorination,” Mullen said.

The municipal pools were pristine by comparison.

The Crotona pool in The Bronx, Red Hook in Brooklyn, Dry Dock in lower Manhattan, Astoria in Queens and Tottenville in Staten Island had water so clean, Mullen said, “they nearly meet drinking-water standards.”

brad.hamilton@nypost.com

