Answer:
Studies have shown that Swimming is 4x more effective than running in regards to burning calories (based upon individuals who had just begun doing exercise, as well as individuals who were accustomed to different types of exercise). The water acts as a resistance, so your whole body has to push and pull to get you through the water. If your goal is to burn lots of calories during a workout, swimming for a half-hour is a great way to do it, and will exercise the entirety of your body.