Prepare your glasses by going around the rim of each glass with a lime wedge to moisten the rim. Place some coarse salt on a small plate and dip the rim of each glass into the salt, turning the glass to coat the entire rim in salt.
Combine simple syrup, lime juice, orange juice, mango nectar and sparkling water in a pitcher and mix with a spoon or whisk until all ingredients are combined.
Fill each glass with ice cubes and pour margarita right on top of ice, filling each glass to just below the salted rim. Garnish glasses with fresh lime slices and enjoy!