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.