Combine the water, cranberries, sugar, and orange zest in a saucepan over medium-high heat. Bring to a boil, then reduce the heat and simmer about 15 minutes, until the cranberries soften and pop.

Transfer the cranberry mixture to a blender, and purée on high for 2 minutes. Strain through a fine-mesh sieve into a bowl, and allow to cool.

Measure out 1/2 cup of the orange juice, and stir into the cooled cranberry purée. Pour into a 9x13-inch metal baking dish, and freeze for 1 hour. Scrape the frozen edges into the center, and freeze for about 3 more hours, scraping it hourly, until the granita is icy and flaky. Freeze for at least 2 more hours, or overnight.

Scrape the granita into serving bowls, and top with whipped cream.