An artist's impression of... guess what? If the poster was printed in 1930, as some sources insist, the liner in question should be diesel-powered Saturnia or Vulcania, built in the mid-20s:
But if the sources are wrong and the artist's vision was a bit more realistic than we tend to think, the ship must be MS Neptunia or Oceania (commissioned in 1932 and 1933, respectively):
Let us ignore the paint scheme and take the sources for granted. It suits our plot.
UPD: After a more thorough research we are 100 per cent sure that the liner in question is MS Saturnia. Proof: original inaugurational cruise poster, 1927