Now, I will start by saying that Titanic needed to be a big box-office hit given its cost. Before its release in 1997, many critics and Hollywood insiders predicted that Titanic would be a box-office bomb. And there were several reasons why it was predicted to be a failure. First off, at the time it was the most expensive movie ever made and was getting compared to the costly Waterworld — which had been released a couple of years before and, like Titanic, was the then–most expensive movie ever made. Even though Waterworld grossed over $200 million at the box office, it was deemed a failure/moderate success ’cause it barely made a profit, and it also became the butt of jokes.

Second, aside from the cost of filming, Titanic also took a long time to film. Going way over schedule also delayed the movie’s release — as it was originally meant to be released during the summer (the blockbuster movie season). Moving its release date to December set off red flags.

Lastly, many on-set stories were reported throughout the film’s production, everything from the difficulty of working with director James Cameron to the PCP poisoning of some crew members. All these things combined added to the “doomed film” narrative.

The movie had a very good opening weekend when it was finally released, grossing $28.6 million. However, that didn’t scream blockbuster movie; for comparison, Men in Black, which was a blockbuster movie that had come out in the summer of ’97, grossed $51 million on its opening weekend. What made Titanic different was that it actually grossed even more on its second weekend (grossing $35.4 million) because of word-of-mouth. Another unheard-of thing was that the movie would continue to gross between $21–36 million every weekend for the next two months, which was mainly due to people doing repeat viewings. The movie would go on to be the No. 1 movie at the box office for the next 15 weeks straight and ignite Titanic-mania, which turned the film into an iconic ’90s pop culture phenomenon. 

Source: https://www.buzzfeed.com/briangalindo/things-90s-nostalgia-today-gets-wrong-or-skips-over