Swimming and fat loss is an interesting topic. On average, elite swimmers tend to have higher bodyfat percentages than runners or cyclists of the same standard. Theories for this include exercising in cold water (less blood flow to subcutaneous fat) and an increased appetite, as has been mentioned in this thread. Also, some subcutaneous fat may be beneficial to swimmers - it aids buoyancy and reduces drag.
HOWEVER, as a method of conditioning/recovery used alongside a strength training program, it is fantastic. Think of a swimming session as the ultimate form of active recovery from a hard training session. All the stretching/mobility/foam rolling work in the world cannot compare to the four swimming strokes performed correctly in terms of improving hip, shoulder and ankle mobility. This is especially true for breast stroke. PLUS it is non-weight bearing, so your joints get a break.
FINALLY, as a form of pure performance conditioning (i.e. not necessarily just fat loss), interval sprints are a fantastic way of getting some work in in the anaerobic zone and improving your lactic acid system (with the added benefits listed above). Plus there is no eccentric element so recovery tends to be less of an issue.
The one thing that no one has brought up yet (and really needs to), is the issue of technique. Running fast, of course, requires attention to technique, but running is a basic human movement, something programmed into our genes. Swimming is not. It’s a fact that most (not all) of the fastest swimmers in the world have very low levels of strength. They have exceptional technique, however.
This is important because the number of calories you can burn swimming is in some sense relative to your abilities as a swimmer. For example, I’ve had the opportunity to teach over a dozen football players how to swim proper freestyle. When we started, most of them were absolutely gassed just doing a single lap; 5-10 laps over the course of half an hour was enough to exhaust them. Their strokes were so inefficient that every single lap was exhausting. Once they learned proper technique, however, they could go for much longer. This would be my one caveat - swimming is great for conditioning so long as you have good technique. It is much less effective of an exercise regimen if your technique is poor.