Swimming an Excellent Choice for Exercise
Swimming is an excellent choice if you want to use this as part of an exercise program or if you suffer from arthritis etc as the water will support your weight and in turn relieves the stress on your joints and this will give you a good workout without putting too much strain for example on your spine, knees and hips. Swimming will tone your muscles up because the resistance of the water against your body and in turn gives the major muscle groups a good workout. This type of exercise will not help with bone density as you need weight training to help with that so don't just concentrate on the swimming get into the gym go cycling, walking, lifting weights that will increase your bone density whether you are young or not so young, try and alternate your programs if possible.
Source: Health-and-Fitness:Exercise