01 - Combine watermelon cubes, lime juice, and granulated sugar in a blender. Purée until smooth.
02 - Taste the mixture and add more sugar if desired. Stir in chopped mint leaves if using.
03 - Pour the mixture evenly into a shallow, freezer-safe baking dish.
04 - Freeze for 90 minutes. Using a fork, scrape the surface to break up ice crystals.
05 - Return dish to freezer. Every 30 minutes, thoroughly scrape the entire mixture with a fork, repeating the process for 3 to 4 hours, until texture becomes light and granular.
06 - Spoon the granita into chilled bowls or glasses. Garnish with additional mint or lime zest if desired.