Puree the watermelon in a blender and measure out 4 cups; reserve any remaining puree for smoothies or ice pops. Whisk in the liqueur, agave nectar, and salt until the agave and salt are dissolved.
Transfer the mixture to a 9-by-13-inch dish and freeze for 4 hours, scraping the mixture with a fork once every hour to create small flakes. The granita can be frozen, covered, for up to 1 week.
Scoop the granita into 6 chilled glasses and serve immediately.