In a blender, puree the watermelon until smooth. Set a fine-mesh strainer over a bowl and strain the watermelon juice, pressing gently on the solids to extract as much juice as possible. Discard the pulp. In a large pitcher, combine the simple syrup with the juice of the lime and mint leaves. Using a wooden spoon, lightly muddle the mint. Add the tequila. Refrigerate both liquids until chilled, about 2 hours.
In bottom of rocks glasses pour 1/4 oz Chambord. Fill with ice. Gently layer 2 oz watermelon juice. Top with 2 oz tequila mixture and stir gently. Garnish with the mint sprigs and serve.