Swimming improves endurance. In one study of sedentary middle-aged men and women who did swim training for 12 weeks, maximal oxygen consumption improved 10% and stroke volume (the amount of blood pumped with each beat which indicates heart strength) improved as much as 18%.
In a study of men who completed an eight-week swimming program, there was a 23.8% increase in the triceps muscle (the back of the arm). My take on muscle mass and swimming is that if you have been doing no resistance exercise at all and you start to swim, you will certainly get more toned and you may even gain mass like the men in this study. But even without the gain in mass, it's well worth the strength and tone that you will almost certainly gain.
Swimming burns lots of calories, anywhere from 500-650 per hour depending on how efficiently you swim (you burn more flopping around than swimming cleanly!) and how buoyant you are (the more body fat you have, the more you float and the fewer calories it takes to swim). Very early and original research on swimming and calorie expenditure showed that swimming, regardless of the stroke, burned about 89% of the calories burned during running and 97% of the calories burned during cycling for the same time period. Stated another way, swimming burns about 11% fewer calories than running but only 3% fewer calories than biking. One important caveat about this data is that calorie expenditure is dependent on the intensity of exercise, and so it's entirely possible to burn more calories swimming than running in the same period of time as long as you swim hard enough, and particularly so if compared to running at light intensity.
I grew up watching 'Superman.' As a child, when I first learned to dive into a swimming pool, I wasn't diving, I was flying, like Superman. I used to dream of rescuing a girl I had a crush on from a playground bully.