Combine all ingredients in a medium mixing bowl and stir until sugar melts. Pour mixture into 9-by-13-inch metal pan (the mixture should only come about 1/4 inch up the side of the pan) and place on level shelf in freezer for 30 minutes. Remove and use a dinner fork to scrape any ice crystals that have formed on the side or bottom of the pan. Return to freezer and repeat scraping every 20 to 30 minutes for 3 to 4 hours. Once mixture is thoroughly frozen, fluff with a fork and allow flakes to "dry" in freezer another 30 minutes before serving. When served, the granita should look like a fluffy pile of dry brown crystals.
Scoop into goblets and top with barely sweetened whipped cream. Add additional citrus zest, if desired.