How to Make It
In a saucepan, combine the water, cranberries, sugar and orange zest and bring to a boil. Reduce the heat and simmer until the cranberries pop and soften, about 15 minutes.
Transfer the cranberry mixture to a blender and puree at high speed for 2 minutes. Strain the puree through a fine sieve into a bowl and let cool completely.
Stir the orange juice into the puree and pour it into a 9-by-13-inch metal baking dish. Freeze the granita 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 for at least 2 hours longer, or preferably overnight.
Scrape the granita into glasses and serve topped with sweetened whipped cream.