In a medium saucepan add the water, sugar and mint leaves and bring to a boil. Once the sugar has dissolved, turn off the heat and let the simple syrup cool.
Remove the rind from the watermelong and cut the watermelon into chunks.
In a blender add the watermelon, lime juice, mint leaves and cooled simple syrup. Puree it all together.
Pour the mixture into a baking dish and freeze for at least 8 hours.
Once frozen, take a fork and scrape the frozen mixture to create the granita texture.