Add the frozen watermelon, lime zest, lime juice, and honey into a blender and blend on high speed until it's silky smooth. If the pieces get stuck, shake the blender and move them around with a spoon to get the blender going again. You might have to do this a few times throughout the process.
500 grams frozen watermelon cubes, 1 lime's zest, 1/2 lime's juice, 1 tsp honey