City leaders, members of the clergy and activists want to dry-dock a floating jail off the South Bronx once and for all.

They rallied Thursday to demand the closure of the Vernon Bain Correctional Facility, also known as "The Boat."

Marvin Mayfield, who was one of 800 detainees on the floating jail 10 years ago, says he spent almost a year there.

"What I remember about that boat was the lack of air," he says. "For 11 months I languished, because I couldn't pay a bail."

And he describes it as a "hell hole."

It was dark. And it was so cramped that inmates slept within arm's length of one another.

"Head to toe," Mayfield says. "It looked, literally, like a slave ship."

It was built in 1992 to accommodate an overflow at Rikers Island, the embattled city jail that Mayor Bill de Blasio wants to close.

It was supposed to be temporary. But more than two decades later, it remains anchored off the southern shore of Hunts Point.

Critics say it costs the city an average of $247,000 a year to keep a single person locked up there.