There are some hardwater species (e.g. E. bleheri) but I'm not aware of any that tolerate brackish water. Perhaps do a search of "salinity" and "Echinodorus" and see what comes up?
It may well look like its going to die but if might then recover - when we stopped CO2 and ferts in our planted tank the BIG swords lost all of their outer leaves and are now health but smaller versions of their former selves.