Add water and sugar to a small saucepan and bring to a boil. Stir until sugar is dissolved and turn off heat. Set aside and add the rosemary. Let rosemary steep for 1 hour. 

Rub squeezed lime over rim of glasses and roll in margarita salt. Set aside. 

To make the margaritas, add 2 teaspoons of the simple syrup, lemon and lime juices (~4 oz), tequila, Grand Marnier, 1 sprig fresh rosemary, and a couple of ice cubes to a shaker. Cover shaker with lid and shake vigorously to combine. 

Add crushed ice to 2 glasses and evenly divide margarita between them. Top with club soda/flavored water and garnish with sliced lime and fresh rosemary.