Bring the sugar and water to a boil in a small saucepan and cook until the sugar has completely dissolved. Let cool.
In a food processor, puree the watermelon flesh. Add a little of the sugar syrup to sweeten, to taste. Pour the pureed watermelon into 2 empty ice cube trays and freeze for at least 4 hours.
In a blender combine the frozen watermelon cubes, more simple syrup, to taste, lemon juice and vodka, melon liqueur and blend until smooth. Pour into 8 frozen martini glasses and garnish with a lemon twist.