Swimming is a kind of exercise. any exercise after eating is harmful. Moreover, swimming is like bath as you come in contact with water. Hence, this is like taking bath as well as exercise. Hence, it is more harmful.
Swimming is a physical activity and we should not indulge ourselves doing physical activities with a heavy or full tummy. If one cannot avoid it, then make sure there is sufficient gap between your food intake and swimming...!
As gulshanji sai,swimming is one type of exercise and doing excercise is really harmful after eating. If you want to do swimming,then you should take some time and have a break of couple of hours and then do swimming.