What about the “mirage” theory that a natural phenomenon obscured the iceberg until it was too late?
Not familiar with that one, or I've read about it and forgotten about it. Not familiar enough with some of the funny optical things that can happen at sea to render a worthwhile comment.
I think the best explanation is that the sea was glassy calm and there were no waves breaking against the ice and pretty much no ambient light until the cast of Titanic's own lights made it more visible.