Swimming is an overall fantastic workout because you have to move your entire body agaisnt the resistance of water.
Swimming builds endurance, muscle strength, and improves cardiovascular fitness. It helps to maintain a healthy weight, healthy heart and lungs. It also tones muscles. It's a whole-body workout, as all the major muscle groups are used during swimming.
Swimming is a stress reliever, and it improves balance and posture. Most importantly, swimming provides a low impact therapy for some injuries and conditions.