It's near impossible to move gracefully while standing up to your chest in water, so from the spectator's point of view, aquaerobics is an ungainly form of exercise. But this is keep-fit, not synchronised swimming, so who cares? And everyone is grinning happily as they splosh about to the music, so it's obviously good fun.
It is also an effective workout. Forty-year-old Christine has lost a stone-and-a-half in the five weeks she's been coming, "and it's the only form of exercise I've ever enjoyed," she adds. Another participant says is hooked after just one session. She says: "I've tried ordinary aerobics, but I could never keep up. This is really enjoyable and not at all competitive." Another great advantage of exercising in water, enthusiasts point out, is that whatever your size, you don't feel self-conscious because most of you is covered.
Having previously taught dry-land aerobics. Samantha, the instructor (who was trained by Glenda Baum, the fitness guru who brought aquaerobics to Britain 12 years ago) is a total convert to the underwater kind. "It's the perfect exercise for people who don't like gyms," she says, "because the water acts like weights, toning your body and helping you build up stamina."
She leads the class from the poolside, shouting encouragement (you need a loud voice to make yourself heard over the music and splashing) and leaping energetically about to demonstrate the movements. She seems to be taking more exercise than anyone else, but of course the others are doing it under water.
The hour-long workout includes jumping, jogging, kicking, and striding in the water, whole-body twists, turns and stretches, underwater punching and balletic movements of the hands and arms.
Most of it is done standing up, although some exercises call for floats. "I'm a cowardly swimmer, but I am confident doing this," declares one mature mermaid.
This class takes place at Putney Swimming Pool, Upper Richmond Road, London SW15 1BL.Tel: 0181 785 0388