For some time, some people thought that because water is generally cooler than our body temperatures, it would be difficult to lose weight with a water workout. Like many old ideas about exercise, this has since been revised: Swimming is now recognized as one of the biggest calorie burners around, and it's great for keeping weight under control.
The exact number of calories you burn, of course, depends on your own physiology and the intensity with which you exercise, but as a general rule, for every 10 minutes of swimming: the breast stroke will burn 60 calories; the backstroke torches 80; the freestyle lights up 100; and the butterfly stroke incinerates an impressive 150.
To boost the calorie-burning component of swimming, consider using intervals in which you work your hardest for short bursts and then recover. One way to structure this kind of workout would be to swim 50 yards (45.7 meters) then rest for 10 seconds, then 100 yards (91.4 meters) with a 10-second rest, then 150 yards (137.1 meters) -- all the way up to 300 yards (274.3 meters) with rests in between. When you reach 300 yards, reverse the pattern [source: Petterson].