If you are doing cardio for the fat burning effect, swimming is not your best choice because of the diving effect. When your face is submerged in water your body naturally starts to conserve 02. Because of this you don't get the full aerobic affect and actually burn less calories because less 02 is burned. For fat loos, weight bearing exercises would be a better choice because they burn more calories and you don't have the diving effect.
Now, if you want to gain or increase you lung and vascular system aerobic capacity, swimming is excellent because of the diving reflex.
But any aerobic training at a medium to intense level will reduce your muscle mass and ability to gain muscle because it is counterproductive to gaining and maintaining type II muscle fibers.