They didn’t consider it ‘unsinkable.’
The fact that the Titanic is so large and powerful that it is unsinkable is made a big deal in the film. The captain believed it, the crew believed it, and the passengers believed it as well. In reality, however, it wasn’t quite like that. Of course, the ship was remarkable, but it was not thought to be “unsinkable,” partly because it would be ludicrous. Why should a ship be unsinkable simply because it is enormous and attractive? This legend arose after the accident and persisted long enough to be depicted in the film.