Director George Sidney had to leave for a few days because of illness, so uncredited associate producer Roger Edens directed the fog-enshrouded "departure" sequence, including William Warfield's performance of "Ol' Man River." That scene has been praised even by critics who hate this version of "Show Boat."
It reportedly took only one take for William Warfield to pre-record his rendition of "Ol' Man River" prior to filming.
The Breen Censorship Office challenged the "miscegenation sequence." They failed because the 1936 film version had it, setting a precedent.
The showboat built for the film (known as the Cotton Blossom) became an amusement park attraction in 1973, after MGM sold many of its props at an auction. Unfortunately, in 1995, it was dismantled and torn apart. For this film, the Cotton Blossom was built on top of a flat-bedded barge so that it could be towed into position by underwater cables for the musical number which opens the film. Even though the Cotton Blossom was built to exact specifications and was fitted with a stern paddle-wheel, the thrust of the paddle wheel would have been too strong to maneuver the boat in the studio lake. Too little thrust would have moved the boat very slowly if it moved the boat at all. Hence, it was necessary to move the boat into position by underwater cables. This underwater towing technique also made it easier for the boat to move into its mooring position at exactly the right moment when the musical number came to an end.
Although Annette Warren dubbed Ava Gardner's singing voice in the movie, Ms. Gardner herself sang her two songs on the MGM soundtrack album.