Swimming is one good exercise for weight loss....yes, it can gain muscles as the other exercises do....but muscles are being formed when you exercise more and when you eat more protein. Weight is being gained when you take more calories than required. So, in able for you to reduce some pounds, you have to exercise and lessen your calorie intake (eat less - take a lot of foods rich in fiber.)

And as mentioned by redtreble CV(cardiovascular)will burn off the calories. I recommend jogging...it is the cheapest and effective CV.