Place sugar and 3/4 cup water in a small saucepan. Bring to a boil over medium heat, stirring occasionally. When sugar has dissolved, remove pan from heat, and stir in creme de menthe, if using. Chill the syrup.
Remove seeds from watermelon flesh. Place flesh in the bowl of a food processor; process until liquefied, about 2 minutes. Add puree to chilled syrup; stir to combine.
Freeze sorbet in an ice-cream maker according to the manufacturer's instructions. Store, frozen, in an airtight container.