Watermelon is like 90 percent water, so you would think jus tembikai is basically free. Problem is the stall almost always adds sugar syrup, even when the fruit is already sweet enough. That habit turns a light fruit juice into a 130 calorie glass.
It is still better than a Milo Dinosaur lah, you do get actual fruit. But juicing strips the fibre, so you lose the part that keeps you full. Honestly eating a wedge of real watermelon beats drinking it, fewer calories and you actually chew something.