Combine ingredients in a blender. You can put in half the watermelon, blend, and then add the other half. Blend until smooth.
Pour into a 9×13 baking pan. Freeze for one hour, or until just firm.
Using a fork, scrape all the frozen ice to create a slushy consistency.
Return to freezer until ready to serve.
You can substiute a handflul of Splenda packets for the sugar