01 - Place watermelon cubes in a blender. Add lime juice and sugar. Blend until very smooth.
02 - Taste and adjust sweetness, adding more sugar if desired. If using, stir in chopped mint.
03 - Pour the mixture into a shallow, freezer-safe baking dish, spreading evenly.
04 - Freeze for about 90 minutes. Using a fork, scrape the surface to break up any ice crystals.
05 - Return to the freezer. Every 30 minutes, scrape thoroughly with a fork, repeating for 3–4 hours, until the mixture is light and fluffy.
06 - Serve granita in chilled glasses or bowls. Garnish with extra mint or lime zest if desired.