Perhaps, if you make a movie with a name like "Titanic", it's only right that you make an absolutely massive error with the time-line. Everybody remembers the iconic scene in the water where Jack is talking to Rose and they say their goodbyes. Fewer people remember a conversation earlier in the film where they're getting to know each other, and Jack casually mentions that his father used to take him fishing at Lake Wassota.
Jack is a man of unusual charm and appeal, and perhaps part of that charm comes from the fact that he's a time traveler. He'd have to be. Lake Wassota was created by a man-made dam in 1917 - some five years after the Titanic sank. We realize there was no Google in 1998 when the film was made, but this sort of information surely had to be in an encyclopedia?