For cranberry granita:
1) 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.
2) Pass the mixture through a fine mesh strainer directly into a metal 32.5 by 22.5-cm 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.
3) Once frozen, scrape the mixture with a fork to create a shaved ice texture. Serve immediately.