Not only does swimming tone the stomach, or abdominal muscles, it strengthens them. Performing swimming exercises consistently helps keep the abdominals in shape while providing an excellent cardiovascular workout.
Treading water in the deep end of the pool provides an excellent abdominal workout, as does paddling while holding a kick-board. These swimming exercises isolates the stomach muscles, forcing them to work in conjunction with the legs.
The flutter kick style of swimming works the quads, calves, hamstrings and glutes as well as the abdominal muscles. It works the abs by forcing them to stabilize the swimmer in the water. In fact, the higher in the water swimmers keep their bodies while performing the flutter kick, the more the abdominals have to work to keep the body afloat.