Purée the watermelon in a blender and measure out 4 cups; reserve any remaining purée 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 9×13 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 one week.

Scoop the granita into 6 chilled glasses and serve immediately.