Here's a link to an interesting article on exercise, and answers the question: Is Swimming a Good Exercise for Seniors? http://blog.intheswim.com/health-aging-seniors-swimming/

The article mentions the following benefits of swimming:

• Swimming increases oxygen distribution to the entire body.
• Swimming helps reduce body fat, tone muscles and increase the lean muscle stamina and endurance.
• Swimming activates all large muscle groups of the body, while other exercises isolate fewer muscles.
• Swimming is easy on the joints, increases flexibility, tones and shapes muscles.
• Swimming reduces the risks of osteoporosis and heart disease and engages a healthy state of mind.
• Swimming lets you exercise without pressure on the joints.
• Can lead to Improved flexibility which can allow for better posture, less soreness and a low rate of injuries.
• Safer way to do Balance exercises.
• Swimming relieves stress and improves the mood.
• Social activity.