On the 17 day diet watermelon is discouraged because it contains a lot of sugar which this diet tries to get people who are on it to cut down on. Foods that are high in sugar or saturated fat are usually not allowed because of the fact that it typically contributes to a significant amount of weight gain.
Although watermelon may be high in sugar, it does still have quite a bit of nutritional value to it for those who are trying to stay healthy. People who want to cut down on their sugar intake though will probably want to limit the amount of watermelon which they consume on a regular basis. Although it may have lots of sugar, it is rich in certain vitemins and nutrients which the body needs to stay healthy and function properly.