The trick with any cardio activity is to keep your heart rate up for at least 30mins. Most people start swimming and find it hard so only do about 15mins worth.. and you haven't really budged the fat.

Avoid swimming like you're in a marathon, just keep a constant pace and if tired, rest for a couple of mins, then get back into it. before you know it, you'll be swimming for 40-60mins straight and losing weight.

That's how I lost my weight, b/c the thought of running makes me cringe plus I have big boobs which are not conducive to bouncing sports!