As stated in an interview with James Cameron one night on the Late Show, James Cameron stated to Jay Leno that there were no passengers listed on the Titanic by the names of Jack Dawson or Rose(I can't pronounce her last name cause it's so long). He also added that it was the screen writer's imagination and story telling that added into the names of these two characters. He also added that if there were passengers unlisted on the Titanic by those names that it would be simply coincidental. But the question that comes to mind is that in the movie, it was said that there was no passenger listed under the name of Jack Dawson. The character Rose then replies to the shipmate, "Well, of course you wouldn't find someone under that name because Jack never existed to those on the ship." So you see there might have been passengers on that ship that were stowaways but when the tragedy happened there would be no records of them and there would be no way for any living soul to actually find about those who were on that ship who "weren't really there".