Combine all ingredients in blender. Start off with a couple of quick pulses to break up the large watermelon chunks. Then process for 20 seconds or so, until the chunks are broken down into a slushy consistency.
Freeze excess fresh watermelon in 2-cup (300 g) packages of 1-inch (2.54 cm) chunks, so you have them on hand for this recipe. Don't run the blender any long than necessary. The goal is just to create an icy slush. If you puree it too long, it could heat up and melt.