My understanding is that people who are overweight are advised to drink more water. The reason, however, is not because it consumes calories, but because water (and, for that matter, anything with bulk but low calories) fills the stomach, preventing them from eating foods that do contain calories. Since the body can process water quickly, and most people exist in a state of mild dehydration, this isn't an unhealthy practice (and may be healthy if you are dangerously overweight).