I'm an exercise-class junkie. Yoga, Pilates, TRX, Zumba - if there's a group of people doing it, I will join in. Perusing the schedule at the downtown Tellepsen YMCA one morning, I spotted an unfamiliar class: Aqua Cycle. That's how I ended up riding a bike in a swimming pool. The underwater trend started in Europe. Tim Fletcher, who leads the classes in Houston, went to France to be certified as an Aqua Cycle instructor. On my first visit, he told our small class how to put