Chest-deep in a swimming pool, no one can see you sweat. No one can see the flab flopping around either, which is another benefit of water aerobics, the fastest-growing form of exercise for the gym-weary boomer crowd. Tricia Moon's classes--ongoing at the DISD pool on Hermosa at Peavy and at the White Rock Athletic Club--combine high-energy aerobics, underwater weight and resistance training, some tai chi, a little kickboxing (so much easier underwater) and even a bit of go-go dancing. Moon keeps classes lively with a steady patter of jokes and encouraging words. Watching her go through the moves (at 43, she's all leg and lean muscle) is motivation to keep pushing when the urge is to dog-paddle. For three summers, Moon has led a popular (and addictive) Saturday-morning class at Rowlett's Wet Zone water park, where water exercisers jog, skip and leap against the strong current of the "river" pool. It's like fighting a riptide, and it's a killer workout. That class starts again next May.