Place a saucepan over medium heat and add the water and sugar; and then mix until the sugar dissolves. Add the sprigs of rosemary and bring the mixture to a simmer, cooking for 5 minutes. Take the pan off the heat and allow the syrup to steep while you make the rest of the drink, about 10 minutes. After it's done steeping, remove the sprigs of rosemary and discard. Note: You could also make this rosemary syrup ahead of time and keep it in the fridge until you're ready to use it.
Next, make the mixture that'll rim the glasses. Combine the sugar, salt and grapefruit zest, rubbing the mixture with your fingers--this will release all of the zest's fragrance. Set aside. Juice the grapefruits, which will result in approximately 1 1/4 cup of juice. Run the juice through a strainer and discard the pulp.
To assemble the drinks, add 1/4 cup of grapefruit juice to a shallow bowl. Dip the rim of the glasses into the grapefruit juice and then immediately dip them into the sugar/salt mixture. Fill the glasses with ice. Divide the grapefruit juice between the two glasses. Pour in two or three teaspoons of rosemary syrup, top with sparkling water and mix the drink with a spoon. Adjust syrup content accordingly. Garnish with a few sprigs of fresh rosemary and drink!
If you're feeling crazy and adventurous, I recommend torching the rosemary leaves with a kitchen torch or over a gas range. Garnish and experience a smokier, more aromatic drink.