There are a number of factors that enter into a discussion of this question. Principally, however, the biggest thing we need to remember is the way in which we know the fate of the ship and so there is massive dramatic irony as we watch the passengers board the supposedly unsinkable ship and consider the interplay of relations between the various members of the passengers on board. The tension of the film is thus greatly related to our enjoyment and understanding of the film. We know the ship will sink, but we do not know which characters will survive and how this tragedy will impact their lives and change their situations--for the better or for the worse.
Therefore, in answering this question you need to think about dramatic irony and the way that it operates in this movie, just as it operates in plays such as Oedipus Rex and Romeo and Juliet. We know the ending, but such texts manage to engage our interest and keep us from getting bored by the way in which the characters respond and react to the tragedy that they face. Even though we know the ending, and how certain elements of the plot will conclude, it is fascinating watching characters who do not share that knowledge and the way that they are forced to react to the gradual revelation of what we already know.