How to Make It
In a blender, combine the watermelon with the sugar, lime juice, salt and 1/3 cup of water. Puree until smooth, about 1 minute. Pour the mixture into an 8-by-8-inch metal or glass baking dish and freeze for 1 hour. Scrape the frozen edges into the center and freeze for about 3 hours longer, scraping hourly, until the granita is icy and flaky. Freeze it for at least 2 hours or, preferably, overnight.
Scrape half the granita into 4 small glasses and top with the tequila and the remaining granita. Serve immediately.