The Houdini move
Although Houdini was able to successfully unlock himself underwater, this does not imply that anyone else can. In the icy ocean waters, Jack and Rose had to open a metal gate for themselves underwater. To what extent was that likely? Not really, though.
Your fingers would have become useless in the length of time it would have taken to fumble in frigid water and unlock a gate with a key. Therefore, for the two of them, this could never have worked.
