Chill watermelon in the freezer for 30 minutes. Put all the ingredients in a blender and puree until smooth. Finish with a squeeze of fresh lemon.
Serve very cold in a chilled glass, and garnish with watermelon or lemon slices.
Put the sugar and water in a small saucepan. Cook over low heat, stirring, until the sugar dissolves. (Alternatively, heat in the microwave.) Store in a sealed container in the refrigerator for up to 1 month.
