Combine juice and sugar in a large bowl, stirring until sugar dissolves. Add oranges and water.
Pour orange mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon sorbet into a freezer-safe container; cover and freeze 1 hour or until firm.
If you don't have an ice cream maker, freeze the mixture in a covered baking dish, fluffing every 20 minutes with a fork as it solidifies to keep the mixture aerated.
Scoop into serving dishes and garnish with bittersweet chocolate.