Cut watermelon in half lengthwise. Using a melon baller, cut into balls and place in a shallow serving bowl.
In a blender, mix the mascarpone with the citrus zest and fruit juices, and the sugar until smooth. If too thick, thin with a little water (it should be as thick as yogurt). Drizzle the mascarpone over the watermelon balls, sprinkle the fruit segments over and serve cool.
Recipe courtesy of Mario Batali