I know magazines are not the gospel but according to Mens Health 30 minutes in the pool doing a slow freestyle burns somewhere around 287 calories, it'd take hours to burn close to that much walking on the treadmill.

The pool is great because of resistance and you are getting a full workout, not just a specific body part. One of my good friends is a swimmer and she doesnt have to stick to a strict diet cause she burns so much when she swims.