In a small saucepan, combine ½ cup of the water with the sugar, lemon zest and mint sprig. Bring to a boil over high heat and cook until the sugar is dissolved, 1 to 2 minutes. Let cool completely, then strain, discarding the solids.
In a blender, combine the lemon syrup with the remaining 2¼ cups of water, the watermelon and lemon juice. Purée until smooth, then strain into an ice-filled pitcher. Divide between glasses, garnish with a slice of watermelon and a mint sprig, then serve.