certain foods DO have negative calories. watermelon is not one of them
there is no scientific proof for this....
BUT
here is a little method i have devised
in order for a veggie or fruit (these are the only negative calorie food groups) to be "negative calorie", first it must be fat free (which fruits and veggies are), and secondly the fiber content must be equal to or more than half of the total carbohydrate content.
so broccoli = 4 gram carb , 2 gram fiber = negative calorie since fiber content = at least half of carb content
i came up with this theory and method on my own through real world binging on health foods.