Combine all ingredients into a medium saucepan over medium-low heat & simmer 10 to 15 minutes.Cool completely to room temp. Strain into a shallow airtight container; Cover & place in the freezer 2 hours.Take a fork & scrape the surface of the granita to form icy crystals (think of a snow cone).
Heat the heavy cream, half-n-half & sugar in a saucepan over medium heat to dissolve the sugar;
Remove from heat & scrape the seeds from the vanilla bean into the cream;
Add the bean pod.Cover & let infuse 30 minutes
Sprinkle the gelatin over the cold water in a medium-sized bowl & let stand 5 to 10 minutes.
Remove vanilla bean from cream mixture & reheat over medium heat;
Garnish the panna cotta with a couple of tablespoons of the granita with a fresh rosemary sprig.