One of the best ways to firm up and trim your body down. Due to the resistance the water has on the body swimming involves all the major muscle groups this allows the body to burn up to 20% more calories than swimming through air. Swimming a few lengths per day will keep you fit and give you an excellent workout. Swimming also has less impact on the joints than say, running.
If you wish to take your swim a little further try picking up the pace of your swim, you can work up to a great aerobic exercise and give your body an excellent workout.
Start off by swimming 1-2 lengths at a time resting between sets if necessary, after you have swam ten lengths call it a day. The next day repeat the process until you can swim five lengths without a break. Progress to ten lengths in by adding an extra length each time you return.