Toning, eurgh horrible word.
Tone=Low body fat+Developed muscles.
Swimming will help your body quite alot, its a good cardio work out and if you push yourself hard enough then the muscle will develop somewhat. As for getting the body fat down, it'll help alot more than the typical female thing is and eat nothing. Work out your BMR, eat slightly above that and do all this, the fat'll come off slowly but it will come off.
Swimming will make you fitter and thinner basically. Though its not so much the volume as the intensity that'll help it, though both are important