Bring the water and sugar to a boil in a saucepan over medium heat, stirring until the sugar is dissolved. Allow the syrup to cool for 5 to 10 minutes, then stir in the juice and Campari and freeze the mixture in a shallow metal pan. Remove from the freezer, stir and crush lumps with a fork every 30 minutes for 3 to 4 hours. The mixture should be firm but not frozen solid. When you are ready to serve, scrape the granita up with a fork to create grainy crystals and granules. Scoop into pretty chilled glasses and serve garnished with fresh mint.