Swimming laps is one of my favorite cardio workouts since it gets your heart rate up, burns calories (236 per 30 minutes), and strengthens your muscles. It puts less pressure on your joints too, so it's a great alternative to running or biking.
Laps aren't the only thing you can use a pool for, though. According to InStyle, actress Julia Roberts got in shape for her new movie Duplicity by using the pool in different ways. Her trainer Kathy Kaehler had Julia doing a mix of running, cross-country ski movements, and arm exercises while in deep water. Kathy says, "Swimming and water exercises are great ways to build up endurance and stay fit."
So after warming up with a few laps, put on a flotation belt such as the Aqua Jogger ($51) or a Hydro-Tone Belt ($50). Do intervals, alternating between running moderately for one minute, and sprinting for one minute. Then do other types of movements in the water that mimic riding a bicycle, cross-country skiing, jumping rope, or jumping jacks. The water adds resistance to your body, so regular movements will challenge your muscles more effectively.