When you replace the usual gelatin with agar, like this healthy dessert does, you're getting the health benefits of the seaweed from which the setting agent is derived.
Combine the water, pomegranate juice, orange juice, agar, and sugar in a medium saucepan over high heat, whisking occasionally. Bring to a boil. Reduce to a simmer and whisk until the sugar and the agar have dissolved completely, about 5 minutes.
Transfer mixture to a large bowl or liquid measuring cup (for easy pouring). Divide grapes evenly among 8 parfait glasses and pour 2/3 cup pomegranate mixture into each glass, whisking the liquid before pouring to be sure no agar has settled to the bottom. Refrigerate until set, about 3 hours.