This is a difficult question to answer. There's a lot of controversy over whether or not swimming is a good exercise to burn fat.
There are a lot of factors involved here. What stroke will you use? How efficient is your stroke? Are you able to swim for at least 30 minutes continuously using your chosen stroke or do you go into an anaerobic state quickly and take rests?
A lot of people will end up short of breath due to stroke inefficiency and basically not getting enough air (mainly in freestyle). I wouldn't consider oxygen deprevation to be an ideal cardio workout even though the swimmer may feel that he's greatly exerting himself.
Even if you can swim efficiently, heartrate is always lower while swimming compared to on-land exercises. This is probably due to the horizontal postion in swimming and the fact that in water there is no load on the spine. Even if you can swim efficiently, you'll probably have to exert yourself more to get the same fat-burning effect. Water automatically cools the body too, so by eliminating sweating it may also result in less calories burned.
I swim once or twice a week and use the bike, elliptical and rower on my other cardio workouts. I wouldn't rely on swimming as a fat burner, but living in the tropics I find it a welcome relief to the heat.