Scoop the insides of the watermelon out and into a blender. Blend until smooth.
Save the watermelon shell by covering it with aluminum foil and placing it in the refrigerator.
Pour the liquid from the blended watermelon into a large baking dish. Mine was approximately 15" by 10". Be sure your liquid layer isn't too thick, not much more than a half inch.Freeze overnight.
Remove the dish from the freezer and scrape up the watermelon with a fork. You want to work pretty quickly scraping up the watermelon and breaking up any larger chunks.
Spoon the watermelon back into the reserved shell and either serve immediately or cover with aluminum foil and store in the freezer until ready to eat.