Titanic/WMG

Jack was a Fish Out of Temporal Water
No previous records about him? Check. Anachronistic attitude and haircut? Check. Interested in things that weren't popular then? Check. Lived his childhood next to a man-made lake in Wisconsin that wouldn't be built for another three years? Check. Evidently, Jack was a time-traveler from The Future that ended somehow in 1912 Europe and was making his way back to America where presumably his time machine had been left.

Why didn't he know the Titanic would sink? Because it didn't in the original timeline. He and Rose caused the disaster when they distracted the lookouts with their night kiss by the bow.
 * Building on this, we can conclude that the entire sinking was the result of the universe attempting to correct a temporal paradox.
 * Killing over 1500 people to correct a temporal paradox? The Universe is a bitch!
 * Better than killing well over a billion humans, uncounted numbers of animals, plants, and other living things, and destroying everything in existence. And Jack is Jack Harkness with Magic Plastic Surgery. Someone stopped the original cause of the Titanic crashing, so Jack had to distract the lookouts somehow. He doesn't drown (permanently) and Rose is now unkillable (but aging).
 * Jack also commented on the Santa Monica Pier, which did exist in 1912 but wasn't yet the attraction it became a few years later. This is just another example of Jack forgetting that he was in an earlier time.
 * In the movie, the Titanic sank because of Jack and Rose distracted the watchmen... Stable Time Loop? He was supposed to stop the ship from sinking, but Rose kisses him and he misses his moment.
 * Not quite this idea but close enough.

Jack was sent back in time with the express purpose of saving Rose
In the year 2035, humanity has been ravaged by a genetically engineered disease. Scientists have access to a time machine, but having failed to stop the bioterrorist(s) that first started it they go for the Plan B: Ensuring that the female offspring of the only inmune individual, a woman known as Ruth deWitt Bukater that died in the 1930s, live to see the modern day. Young convict Jack Dawson volunteers to the mission of travelling back to 1912, stop Ruth's daughter Rose from committing suicide from jumping off the Titanic two days before the sinking, make sure that she boards a lifeboat during the sinking and, just to be sure, to seduce and impregnate her. Jack will secure his own survival by clinging to the poop rail as the ship goes down and boarding a piece of floating wood that central control knows will pop out after the ship goes down.

Unfortunately, Rose has an idiot moment and decides to go back on board as the ship goes down. Jack has then to craft a new plan and give her his spot on the floating wood, sacrificing himself for her and the Future of mankind. Although he might have failed to impregnate her, his action and his speech to encourage her to have a long life and lots of grandsons was successful: her inmune offspring reached the late '90s, and so a cure could be created and humanity was saved.

The guy from the search crew is right and Rose is lying.
Rose Dewitt Bukater actually did go down with the ship, and Jack Dawson never existed, hence the story of the card game. Rose Dawson Calvert, a failed actress, had always been interested in the Titanic. Realizing that she was a dead ringer for the girl who died on the ship, she stole a name that would suit her and made up an elaborate story for how she came into possession of the necklace (which is actually a clever fake) for one last attempt at fame and fortune. Eventually she decided that her fake necklace would never pass for the Heart of the Ocean under close inspection and threw it away. That night, she had a dream about the Titanic and imagined what it would be like to be the deceased Rose Dewitt Bukater in heaven with Jack forever. In actuality, Jack was a figment of her imagination, and the real Rose died for unknown reasons.
 * This could also explain why Jack talks about places that don't exist until after the sinking of the Titanic, she could have made an oversight when creating Jack's back story.
 * How did she know about the Hockley family?
 * Maybe she did her research? She probably knew that if she were to be taken seriously she would have to have some sort of idea what she was talking about, so it would make sense that she looked into the history of Rose, her family and the Cœur de la Mer.

Jack doesn't die at the end...
He becomes Aquaman!
 * So it was him who called the dolphins to save the survivors... oh, nevermind.

Rose was a self-serving sociopath...
She meets Jack and suddenly doesn't care about Cal any more and doesn't care that this affects her own mother. Later, when she's floating on the hunk of wood, she doesn't even bother to let Jack share the wood with her. Instead, she makes him float in the freezing water. They exchange words about never letting go, but she only hangs on for a few minutes before letting him sink into the ocean without even checking his pulse. Finally, in the end, she drops that necklace into the water when she could have given it to any number of deserving people, including her daughter.


 * Rose agreed to be engaged to Cal but didn't care about him. She was marrying him for his money, which is pretty self-serving -- though you have to be unusually audacious to consider the alternatives. Her mom basically tells her to grin and bear it -- her mom needs the money, too. Throwing the necklace into the ocean satisfies the life-long need to let go; but couldn't she "let go" in another way, even taking it to a pawn shop and saying "I'm letting go of the past..." like normal people do?
 * She agreed to be engaged to him because she was an upper class girl in the early 1900s and he wasn't absolutely unbearable. And frankly, that necklace was hers and she had the right to do whatever she wanted with it. If she'd requested to be buried with it, would that also make her a self serving sociopath?
 * No time and place in history has managed to arrange marriage customs in a manner that did not leave some people hurt or pressured or whatever, so making a bogeyman out of early 1900's mors edoesn't fly; and being from the upper class is not generally considered an extenuating circumstance.
 * The thing is, that necklace was presumed lost in the wreck. She had it all the time, she knew it was wanted, she talked the guy who was hunting for it out of hunting for it, and then she dropped it into the wreck. It's like if someone had hidden a genuine original Picasso in the attic, which we only know existed because we have a photograph from before it was hidden, and the person decides to destroy the painting rather than let it be auctioned off after death... She may have the right, and he was a treasurehunter, but it's still inconsiderate.
 * And at the end, when she dies, she is back with Jack and all the people from the Titanic, who she could only have known for like a week. What, did she care more about a guy she knew for a week than her husband with whom she had children and grand-children?!? Yes, he did change her life forever, but still...
 * I always thought it was just dreaming. It certainly didn't match any definition of heaven I could think of.
 * "I'll never let go" doesn't mean "I won't let go of your hand." It means "I'll never let go of hope and give up on life."

The famous pair of boots found by the Titanic's wreck belonged to Jack.
Do a Google Image Search for "Titanic Boots". There's a famous photo of a pair of boots right next to each other that seem to resemble the pair that Jack wore in the film.
 * Might have been intentional on James Cameron's part?

== Rose does die in the end, and goes down and meets everyone as per the end of the movie. Then it turns out that all those souls can't "move on" and are trapped on the wreck, needing to be Rescued From Purgatory. ==
 * They're all clapping because Rose is one of the things they need to free themselves.
 * Or maybe they're free when Rose arrives?
 * Or maybe the kiss between the two lost lovers freed them? Which is why they didn't clapped when Rose arrived, but rather when the two of them (Jack and Rose) kissed?
 * The alternative (that this is the afterlife FOREVER and the Titanic is their heaven, which begs the question why Cal is clearly visible in the background) certainly sucks, especially if you were one of the crew ("Yay, I get to open doors and 'yes-sir-yes-ma'am' to self-serving jackasses forever for the sin of having been a crew member on the Titanic!")
 * That could of course be Cal's punishment(he certainly doesn't look happy). Plus the "crew members" likely wouldn't have to do anything they found drudging in Titanic-heaven, only good stuff. More likely they'd all move on to where they wanted to go, or BE the upper class, or partiers, or whathaveyou....

The film's adaptation of the Titanic's sinking is wrong.
New research into how the Titanic actually "Split" shows she couldn't have risen out of the water that far and break downward. Instead, researchers have recovered evidence from parts of the keel to suggest she split in more of an inverted fashion as opposed to the film's portrayal of the "splitting".
 * What about this is a WMG?

Rose got pregnant with Jack's baby.
Her granddaughter has inherited Jack's blond hair.
 * A nice idea, but near the end Rose tells her granddaughter that she never mentioned Jack to anyone, "even your grandfather". Rose married after the crash and never spoke of her lover again, which would be hard if she had the guy's kid to look after. Then again, most of what she's having to do after the Titanic sinks will be hard, seeing as she just rejected her social class outright by faking her death.

Mrs Pennyworth is...
Mother

The bullet the Master-At-Arms is seen rolling down the table is his totem.
He's hoping against hope that he's just stuck in a dream.
 * Operating on the theory that Inception pics up where Titanic left off (Jack downs in ocean, wakes up on beach in Limbo) This sinking of the Titanic very well could be a dream, which explains how Jack grew up at a lake that didn't exist when the Titanic sails

The Heart Of The Ocean has the power to grant wishes
Aboard the research vessel, Rose learns from Brock Lovett that part of Le Cœur de la Mer's elaborate, mysterious history involves a legend that it will grant the wish of the person who returns it to the ocean. Rose ultimately does, and wishes to be reunited and restored with Jack and all the voyagers in the unsunk Titanic, to all exist happily ever after.