The motions involved in swimming combined with the dynamic tension created by keeping yourself afloat make for a whole-body workout. And when you exercise in water, your body spends less energy to stay cool, which allows more blood to flow to the muscles, letting you exercise longer and harder.
Your lats, triceps, and deltoids, as well as your glutes and hip flexors, propel you, but your core stabilizes you.
Amp It Up
Scott Berlinger, of Full Throttle Endurance in New York, has his triathlete clients swim at a comfortable pace for 80 strokes (40 strokes with each arm), then speed up for another 20. Keep it up for 30 minutes, and eventually for 45. Do it in a lake or the ocean and you'll deal with the natural resistance of currents and go without the short rests that come from turning around in a pool.