Goodness, when was THE MOVIE, 1992? Surely if those people are still fly fishing, they've joined a club, been to a class or read a book or two.
I do agree, good manners should be posted; for rookies to read-to avoid embarrassment; and so you and I can just point; or ask Mr./Ms Rude, to please read the Rules of Etiquette posted at stream side. I think that's a great idea, and should make confrontations rare, therefore making it easier for park employees.