Losing fat is more in the calories. You have to lose fat to have "visual" abdominal muscles.

As far as swimming vs. running goes, there's nothing wrong with doing both, as in choosing on certain days one in particular. When your body becomes tolerate to running, it's essentially a more developing method to enhance heart rate by changing in speeds, seeing we do run for "cardiovascular exercise" and the faster the heart beats, the more calories you use as energy.

Running would be more beneficial to the lungs and heart, swimming has more resistance and therefore could be better for muscular endurance, you'd essentially develop an overall more effective cardiovascular workout by doing both, doing both would help you more than doing just one.

When it comes to weight loss, it's all in the calories.