No Jack and Rose were completely fictional characters. When James Cameron pitched the film he envisaged it as Romeo and Juliet on the Titanic. You may find this article interesting, The Real Jack Dawson.
In the 1953 "Titanic" the main characters were mainly fictional.
They might have been based on real passengers though.
The real Jack Thayer was a young man who was a tennis player and "Gifford Rogers"(Robert Wagner) was a young man who was a tennis player but there the similarity ends.
The character "Maude Young" (Thelma Ritter) is said to be "loosely based on Mrs. Margaret Brown."
James Cameron might have based his "Jack and Rose" characters in the 1997 "Titanic" on the equally fictional characters "Gifford and Annette" in the 1953 "Titanic". But at least "Gifford" was a higher class college student who "had to get his blue surge suit pressed for dinner" and not borrow a tuxedo like "Jack " did ! LOL
Many of the characters in "A Night To Remember " (1958) are said to be based on real passengers .