Come July 4th, the city’s hottest new destination could also be the coolest – a floating swimming pool off the Brooklyn Heights waterfront, complete with gorgeous harbor views and a manmade beach.

The 25-meter pool, carved into an old barge, will lower its gangways between Piers 4 and 5 near Joralemon Street for a 10-week trial run this summer.

Sunbathers can also soak up the rays on a one-acre makeshift beach, complete with rentable umbrellas, beach chairs and hamburger and hot dog stands.

“It’s going to be the coolest spot in Brooklyn!” raved Borough President Marty Markowitz.

The seven-lane “Floating Pool Lady” is the brainchild of author Ann Buttenwieser, who was researching a book when she stumbled on references to the “floating baths” popular from the early 19th century until about World War II. She now runs a foundation dedicated to repopulating city waters with floating pools, and this is her first.

The pool will be 4 feet deep and hold up to 175 people. It will be open daily from 11 a.m. to 7 p.m. The beach can hold 1,000 people.

Both are preludes to the planned 85-acre Brooklyn Bridge Park, to be built mostly on the massive piers jutting out of the shoreline from Atlantic Avenue to just north of the Manhattan Bridge.

Critics argue there’s too much luxury housing planned for the park, and not enough recreational activities – and some say the floating pool is just a temporary ruse to shut them up.

“A pool that can just be towed away in the middle of the night, that is my concern,” said Roy Sloane of the Brooklyn Bridge Park Defense Fund. “I want a real commitment to recreation – I don’t want a snow job, or a sand job.”

The beach will be modeled after the successful Water Taxi Beach in Long Island City, Queens, with one big exception – no alcohol.

“We made a decision not to try to get a liquor license,” said Harry Hawk, a partner in the Brooklyn burger joint Schnack, which is operating the hot dog stands. “It would be a nice thing to do, but it’s all about making small steps and seeing how everything works, how many people we attract.”

From the Borough Hall subway station, it’s an eight-block walk along Joralemon Street to the pool and beach. A shuttle is also planned.

heidi.singer@nypost.com