When Xtreme Fitness opened its new aquatics center in March, it started offering classes in what may be the next fitness revolution. Hydroriding is basically a cross between spinning and swimming: riding a stationary exercise bike in waist-high water. The water element means there's zero impact on joints and people stay cool so they can exercise longer, helping both endurance trainers and those needing injury rehab. "There's really no one who can't do it," says the gym's group fitness director, Shelia Erwin. Xtreme Fitness has ten Italian-made stainless-steel hydrocycles that can be adjusted to operate like regular or recumbent bikes. They're so popular that the gym has to charge its own members an additional fee for using them. Xtreme offers 45-minute classes on Monday, Wednesday and Friday ($5 a session for gym members, $15 a session for nonmembers). It's the best way to make sure that you look good in a swimsuit.