"In the case of Show Boat, the word 'n------s' as the very opening word serves stunningly to shock an audience from its complacency, to consider (at least subconsciously) the servile conditions to which southern Negroes were subjected a century ago. The word is therefore an indictment against those times and conditions, not against the Negro race.
Thus, the progression of euphemistic alterations to which this opening line has been subjected is almost ludicrous. First it was "N------s all work on de Mississippi"; in the 1936 film it was "Darkies all work on de Mississippi"; in the 1946 revival it was "Colored folks work on de Mississippi"; in Till the Clouds Roll By it was "Here we all work on de Mississippi"; and by the 1966 revival nobody worked on the Mississippi, because the opening Negro chorus was omitted altogether."
(There was, of course, controversy around the use of the n-word lyric option for that 1988 recording, as covered briefly on the Wikipedia page for Bruce Hubbard, who played Joe. Meanwhile, the 1993 Toronto revival used "Colored folks".)