Why should water that is high in mineral salts affect filter coatings any more than distilled water? In most of the situations that I've been in when my cameras have been sprayed with seawater or mud the chances of finding distilled water have been nil. It's more important to get the stuff off quickly in the freshest water you have available.
All this talk of seawater makes me wonder how many sailors there are on the forum. I enjoy sailing, and have a little yawl built in 1896. Bill Tillman, who I met a few times, is probably my all-time hero and role model. He was a bit more talkative than I am, though.