Well, I suppose we shouldn't lose sight of the fact that it's a fictional account of the sinking and thus allow Cameron some dramatic licence in places. I don't know which parts of the ship would've been off limits to passengers but some of the most visually astounding scenes took place on those railings so I'm happy to allow lapses in realism where good quality direction can replace them.
I'm not sure whether basing the story on a real person would necessarily have improved the film. While J&R's romance was a cliche from start to finish, I followed their plight sympathetically as a viewer and the occasional scenes depicting other passenger's torments (especially the Irish woman reading her kiRAB the final bedtime story) were sufficient in giving me a flavour of what the real suffering would've been like.