You can make this slush with no sugar at all, for an almost calorie-free treat.
Mix the watermelon and sugar together in a nonreactive bowl, and let sit for 30 minutes or more to draw out the juices.
Crush the ice cubes slightly, and put in a blender. Add the watermelon meat and juice and the lime juice, and process on low speed until ice is thoroughly crushed. Add the Watermelon Ice, and blend until smooth. Add water to thin it desired.