What if you could melt fat, tone your body and feel like a kid again—all in just 10 minutes a day? Fitness legend Denise Austin, 69, says you can, and the secret is as simple as jumping in the pool.

Aerobic exercise, or cardio, elevates your heart rate with movements that activate your body's large-muscle groups. When done in the water, this type of workout offers many proven benefits, including ...