Jump in the pool, get fitted on a stationary, mostly submerged bike and start pedaling.

That's how a workout at the boutique fitness studio Aqua (78 Franklin St., 212-966-6784, aquastudiony.com) begins.

Combining the gentle resistance of swimming with the high-intensity burn of spinning, aqua cycling is one of the latest "it" workouts in Manhattan's ever-expanding fitness scene. And just recently, the only studio specializing in this type of exercise in the city -- and likely the country -- has expanded their offerings to include four types of classes for women and three weekly classes for men.

But why should New Yorkers give aqua cycling, which is already a hit in Europe, a try?

"People see amazing results," said Aqua's founder Esther Gauthier, who first took an aqua bike class in Paris two years ago, before becoming hooked on the workout, which can burn 600 calories a session. She saw an untapped market in New York City, where she's lived for seven years, and decided to open a studio of her own.

Since April, when Aqua started offering classes, Gauthier says clients have seen changes like weight loss, body shaping, the improvement of health issues and perhaps most enticing, the loss of stubborn cellulite. Plus, being in the water makes for a low-impact sweat session perfect for anyone from pregnant or postnatal women to endurance athletes.

"You don't have to suffer to get a good workout," she said.

Laurence Michiels, a Midtown-based restaurant owner, started taking Aqua classes at the urging of her chiropractor just over a month ago. At that time, she says she was experiencing knee and joint pain, possibly caused by taking too many spin classes. Now, she says she's a regular, logging about two classes per week.

Asked how aqua cycling compares to standard spinning, she offered her thoughts.

"I think it's as hard, if not harder," she said. "It's mind and body cleansing. And toning."

Julia Mau, a first-timer, agreed. Visiting from London for a modeling job, she says that she plans on incorporating the classes into her regular fitness routine of CrossFit, yoga and running.

"It's exhausting but I still feel relaxed," she said. "It's really replenishing."

The amenities of Aqua -- like small class sizes (15 people max) and the spa-like space (think white walls, high ceilings and lots of votive candles) -- come at a price. Although the intro session is $34, which is comparable to other boutique fitness classes in the area, each class thereafter is $40.

But with about 50 classes per week in a niche untapped by other New York outlets, plus forthcoming offerings like a juice bar, a yoga studio and by-appointment face and body treatments, Aqua seems poised to remain part of the fitness landscape. Gauthier even said that her goal is to have additional studios in New York and across the United States, although she has no specific plans to do so yet.

If you decide to give one of the classes a try, be that the 45-minute interval, strength or restorative class or the 60-minute weekend challenge, you might consider clearing your calendar for the rest of the day.

"You definitely want to take the afternoon nap if you take this class in the morning," instructor Zack Schares said.