Cut a wedge of lime and run it around the rim of a margarita or low-ball glass. Pour coarse salt onto a plate and dip the rim of the glass in the salt.
Fill a cocktail shaker half-full with ice. Add tequila, lime juice, and Swerve to the shaker. Seal and shake well until outside is frosted, 10 to 15 seconds.
Add ice to the prepared glass, then strain margarita into it Top with sparkling water and stir. Garnish with a lime wedge.