01 - Place watermelon cubes in a blender. Add lime juice and sugar. Blend until completely smooth.
02 - Taste the mixture and adjust sweetness by adding more sugar if desired. If using mint, stir in the chopped leaves.
03 - Pour the mixture into a shallow, freezer-safe baking dish and spread evenly. Place in the freezer for approximately 90 minutes.
04 - Remove the dish from the freezer. Using a fork, scrape the entire surface thoroughly to break up any ice crystals that have formed.
05 - Return the dish to the freezer. Every 30 minutes, remove and scrape thoroughly with a fork. Repeat this process for 3-4 hours until the mixture has a light, fluffy, granular texture throughout.
06 - Scoop the granita into chilled glasses or bowls. Garnish with additional fresh mint or lime zest if desired. Serve immediately while still icy.