Place all of the ingredients into a cocktail shaker and shake until the granita melts completely. Pour into a martini glass and serve immediately.
Place the water, cranberries and sugar into a small saucepan, set over medium-high heat and simmer until the berries begin to pop, approximately 7 minutes. Remove from the heat. Puree with a stick blender, blender or food processor, for 1 minute. Pass the mixture through a fine mesh strainer directly into a metal 13 by 9-inch pan. Do not press on the skins. Simply allow the juice to drip through the strainer. Add the zest and stir to combine. Place in the freezer until set, at least 6 hours and up to overnight. Once frozen, scrape the mixture with a fork to create a shaved ice texture. Serve immediately.