Swimming is a great way to tone your legs for many reasons. One is that the kick requires use of muscles from your hips and thighs to your ankle so you can get a workout that uses all the muscles of your legs. Another great reason is that the water relieves you of a majority of your body weight so it is a easier initial workout if you are just beginning a workout regimen. This also makes a beginning swimming workout easier on your muscles and joints. Another perk is that swimming is low impact so a swimming workout is easier on your joints than a working that involves something like running.