Thank you for all of these suggestions. I appreciate your help!
I have also looked into that and haven't been able to find other films besides those just mentioned.
There is an old (1937) Darryl Zanuck film, "The Slave Ship". I believe this was the film I saw as a child that has a memorably horrific scene of slaves being cast overboard on a chain. Beyond that I have little memory of it, but there is some discussion of the film in Ellen C. Scott, "Cinema Civil Rights: Regulation, Repression, and Race in the Classical Hollywood Era" from Rutgers UP, which makes is sound relevant to your query. Wm. Faulkner was involved in the writing, which is interesting in itself.