In a small saucepan, bring the water to a simmer with the sugar. Stir over medium heat until the sugar is dissolved, about 1 minute. Set aside to cool. In a blender, puree the watermelon until smooth. Strain the watermelon juice into a bowl using a fine-mesh strainer, gently pressing on any remaining solid pieces to extract as much juice as possible. Discard the pulp. In a large pitcher, combine the sugar syrup with lime juice, blueberries, and 3/4 cup of lightly packed mint leaves. Use a wooden spoon to lightly muddle the mixture. Add the watermelon juice and tequila and stir well. Refrigerate for at least two 2 hours. Once chilled, divide among 8 tall ices filled with ice. Garnish with mint sprigs. Serves 8. This recipe is courtesy of chef Bobby Flay.