Cut the watermelon into 1-inch cubes, removing the seeds as you go. Place the cubes into a colander set inside a bowl. Stir the cubes gently to extract juice without breaking up the cubes. You should have at least 8 ounces of juice. Put the watermelon cubes on a tray and freeze until solid—about and hour.

Mix the syrup, lemon juice, and lime juice with the watermelon juice. To serve, divide the frozen cubes among 4 glasses. Add the tequila, then the liqueur, and then the juice mixture and stir. Garnish with the mint leaves.