Swimming is an excellent form of exercise, providing a good cardio workout and toning all the muscles in the body. The heart is strengthened and the delivery of oxygen to the muscles is enhanced.
Swimming uses more muscles and burns more calories than other cardio exercises.
For the person recovering from injury or sickness, or who is too frail or weakened for a high impact workout, swimming can be the perfect way to get some exercise and to relax.
"It is very important to regulate your breathing while swimming. You will need to adapt the way you traditionally breathe during exercise to allow your breathing pattern to be timed with the mechanics of your swimming stroke. Finally, many people like to use a heart rate monitor to keep an eye on the effort they are putting into the activity. Since swimming has such low impact sometimes we forget to pay attention to how hard we are working our bodies and it can put you in a dangerous situation. Tracking your exertion with a heart rate monitor is a great way to ensure you are working at optimal levels to meet your personal fitness goals."
http://astronutrition.com/blog/benefits_swimming