The best calorie burning cardio exercise, as far as I'm concerned, is the rowing machine (erg). Given that it is a weight bearing exercise (another reason swimming sucks for fat loss) and that it uses pretty much every muscle group you have, you burn calories even better than running. It is also a good strength/muscle building exercise, for as far as exercises go. The one thing that needs to be taken into consideration, however, is technique. Look it up online, or find a trainer that can show you how to do it. Inefficient erging is just that- inefficient. It is also a good way to injure yourself.