In a small saucepan, heat the sugar and lime zest with the water until the sugar is dissolved. Remove from the heat and let cool.
In a blender (I used this this Cuisinart food processor I bought on Amazon), blend together the syrup with the watermelon, lime juice, and tequila.
Strain the mixture and then pour into a 9x13 deep dish that can go in the freezer (like this one on Amazon).
Freeze the juice mixture for 2 hours, then stir/scrape with a fork. Freeze for another hour, stir/scrape with a fork. Repeat until you have a granular watermelon granita.
Store in the freezer, covered, until you are ready to serve. Serve the watermelon granita as is or over the white chocolate pots de crème (see recipe below).
Notes
Makes 4 cups of granita approximately
Feel free to use more lime juice and less water, and play with the ratio of watermelon to sugar and lime juice to get the flavour you want.
Beware of using too much alcohol as alcohol will lower the freezing point and too much may result in the granita not freezing properly or melting too fast.
Initially, it will take a long time to partially freeze this volume of juice, but then as you repeat the process, you will need less and less time between scraping to get the right granular texture.
Flavour variations:
Citrus: this works with oranges, lemons, or limes
Ginger: infuse the syrup with a little fresh ginger, grated!