Everyone loves a crazy fan theory, and this one is up there with the best of them.
The theory was first proposed by a guy, known only as Matt, on Facebook. It hypothesizes Jack from "Titanic" was a "time traveler [who] was only there to save Rose from committing suicide and altering the timeline."
Early in the film, Jack (Leonardo DiCaprio) stops Rose (Kate Winslet) from jumping off the back of the boat to a watery grave.
But, the fate of the Titanic would have been very different had Rose actually leapt to her death.
This may seem ridiculous but think about it for a moment. If Rose jumped to her demise then the ship would have stopped to look for her. The temporary delay would lead to warmer weather and the Titanic would have never hit the iceberg. This is also why Jack made it a note to spend so much time with her, his job was to ensure her survival.
There's more evidence to support this claim. There are quite a few factual inaccuracies and anachronisms in the move that point to a time-travel scenario.
For instance, the theory states,
Jack doesn't have any currency in this time period so he has to gamble in order to get the ticket in the first place.
If he was really from the future, he may not have had money he could legally use at the time.
Moreover, Jack tells Rose about a time he went fishing at Lake Wissota in Chippewa Falls, Wisconsin. But, Lake Wissota is a man-made lake that wasn't built until five years after the Titanic sank.
Similarly, Jack tells Rose he'll take her on a roller coaster on the Santa Monica Pier. Yet, the pier wasn't built until 1916, four years after the Titanic went down.
Lastly, Jack's appearance is chock full of anachronisms. His hairstyle is not of the time, and his rucksack wouldn't become commonplace until the 1930s.
Sure, these may just be oversights on the part of James Cameron. Or, they may point to an interwoven James Cameron film universe in which Rose is Sarah Connor's grandmother.
You be the judge.