Question: Just a quick one: Why the hell didn't Rose just move over on her door to give Jack some room? And why didn't Jack take the piece of wood from the frozen guy with the whistle after he had died?
Answer: In an episode of Myth Busters, they checked to see if Jack could've actually fit on the board and survived. Their first result stated the the movie was correct; there was not enough buoyancy to keep them both afloat. After some thinking they decided to tie Rose's life jacket under the board to increase the amount of buoyancy, and sure enough the board did float, but it's not unreasonable that that wouldn't have occurred to Jack and Rose. When they consulted James Cameron about the results he simply stated, "I think you guys are missing the point here. The script says Jack died. He has to die. So maybe we screwed up and the board should have been a little tiny bit smaller, but the dude's goin' down.