Watermelon Granita

1 cup water
1/3 cup sugar
2 tablespoons crème de cassis
about 3 pounds watermelon, seeds and rind discarded and the flesh chopped
1 tablespoon fresh lime juice

In a small saucepan stir together the water, sugar, and crème de cassis, boil the mixture, stirring, until the sugar is dissolved, and simmer it for 5 minutes. Transfer the syrup to a bowl and chill it, covered, until it is cold. In a blender puree the watermelon in batches, forcing it as it is pureed through a coarse sieve set over a large bowl, pressing hard on the solids. I don't really know how much watermelon I used, but it was probably roughly 5 cups of puree that came out of the blender. Stir the syrup and lime juice into the watermelon puree, and transfer the mixture to an 8-inch-square metal pan. Freeze the mixture, stirring and crushing the lumps with a fork every 30 minutes, for 2 to 3 hours, or until the granita is firm but not frozen solid. Scrape the granita with a fork to lighten the texture and serve it immediately. I was going to garnish with a sprig of mint but forgot....

Slightly modified from this epicurious post.