Actually there are several that can deal with brackish and one that prefers light brackish and can get by in FW for extended periods (Gymnothorax tile), and one that though impossible to find for sale is more common in freshwater and light brackish than is more saline waters it even has a couple landlocked breeding populations (Gymnothorax polyuranodon) here are pics (from fishbase) the top is pretty commonly availabe and is the G.tile sold as a freshwater snowflake moray, the other is the polyuranodon which I have never seen offered.
