Swimming does burn calories at a rate of about 3 calories a mile per pound of bodyweight. If you weigh 150 lbs. and it takes you 30 minutes to swim one mile (1,760 yards or 1,609 meters), then you will be using about 900 calories in one hour. However, many swimmers do not swim that quickly, and many cannot swim for that distance or duration.
One thing swimming is not good for is losing a lot of weight. The on-line Merck Manual explains that swimming is not the best way to lose weight due to the cooling effects of being in the water. While you do use a lot of calories swimming, once you get out of the pool much of that calorie burning stops. Doing land based exercise like running or cycling may use about the same amount of calorie per hour as swimming, but once you stop exercising the land-based workout usually leads to continued increase in calorie use for as long as 18 hours after the workout. Why? Because when you are in the pool you don't heat up as much as you do on land, and your body does not have to work to cool you down as much once the exercise session concludes. Swimming does exercise almost the entire body - heart, lungs, and muscles - with very little joint strain. It is great for general fitness, just not a great way to drop excess pounds.