What if Jack Dawson was really some guy from the future, sent back in time to keep Rose from killing herself? And what if his efforts drastically altered the course of Titanic's fate? One Titanic fan came up with a pretty clever theory about the truth behind Leonardo's wandering character.
Jack from Titanic was a time traveler that was only there to save Rose from Committing suicide and altering the timeline. 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 have 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.
The above half of the theory indicates that Jack's main purpose was to save Rose from committing suicide, which he does. But the theory goes on to imply that in saving Rose, Jack prevented Titanic from having to slow down in order to search for Rose. Had the Captain halted the ship to do a search, maybe it wouldn't have been quite so cold when Titanic eventually arrived at the location of the icebergs. Or maybe the water wouldn't have been so still at that point. Conditions, at any rate, would've likely been different enough that perhaps the ship wouldn't have struck that iceberg. In an alternate timeline, Rose's suicide happens and perhaps Titanic makes it to the U.S.
Let's look at the evidence - 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. He mentions fishing in Lake Wissota, which is a man-made lake created in 1917, five years after the Titanic sank. His haircut seemed completely out of place for the era, furthermore that rucksack wasn't common until the late '30s. He claimed he will take Rose on a roller-coaster on the Santa Monica Pier, which wouldn't be built until 1916. How else would Jack have knowledge of these future places?
The evidence suggests that Jack has awareness of places that don't exist yet in this time period. Also his bag and hair are wrong for the time period, apparently.
If we want to go deeper into the Terminator plot -- and this is where I'm throwing my own layer of complicated madness into this bizarre Titanic-Terminator plot collision -- maybe future-Jack was sent back after a terminator went back in time with the intention of killing Rose. But perhaps instead of killing Rose outright, the terminator decided to plot a more elaborate scheme to get Rose to die, by killing Rose's father and leaving Rose and Ruth with "a legacy of bad debts hidden by a good name." And Rose's desperation was all part of the machine's much-more-subtle plan to alter the future, forcing Rose into a situation where she had to marry Hockley in order to "insure our survival," as Ruth put it, thusly sending Rose into a desperate attempt to end her own life. Had she succeeded, no one would have ever suspected the machines, right? Just a woman in a pretty dress jumping off a ship. It'd be a very under-the-radar approach on the part of the machines, but they would have learned their lesson from the original Terminator's Sarah Connor-killing spree, which was all over the news. Too far-fetched?
Jack is a nickname for John after all. Not suggesting that Jack is John Connor, because gross, but he could've chosen the name and the hair as a nod to the great John Connor... right?