I don't think a RES was meant to live in brackish water, although given that they're in many parts of the work, some have no doubt adapted to living under such conditions.
I wouldn't keep a RES in what I'd consider to be brackish water on the assumption that the shell will be healthier. That said, some people do use a small amount of aquarium salt in the tank (follow directions on the box if you chose to try it) as a preventive measure to help keep up the water quality/keep bacteria down; this would not hurt your RES. I'd use a salt bath (stronger salt content) only for cases of mild fungus.
Use aquarium salt or kosher salt to be safe.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-