I think the chief reason for why this film flopped and is often so reviled is a simple one. It absolutely failed to do justice to what was an outstanding novel of suspense and action which launched Clive Cussler's Dirk Pitt into a signature character of popular fiction on the same level of James Bond before and Jack Ryan later. I defy anyone to read the original novel which has moments of suspense (the murder of a submsersible crewman by a Soviet agent during one of the dives) and action (the Soviets board the Titanic during the height of a hurricane and Pitt has to survive a helicopter crash on the deck) and also doesn't have that annoying element of 1970s moral equivalence regarding the Cold War (in which America's quest for something vital to her interests can not be seen as a straight-ahead case of the good guys winning. All that handwringing about building "byzanium bombs" is nowhere to be found in the novel, and the ending is different. Let's just say at the risk of giving too much away, the digging at that location at the climax proceeds without any hesitation) and tell me the film did justice to it.
I have gone back and read the earlier draft of the script from 1977 when Stanley Kramer was attached to the project (and which is the source of the "adaptation by Eric Hughes" credit) and it looks like the rumors about wanting Steve McQueen for Pitt at the time were true because Dirk Pitt, who in the novels is younger and bolder, is presented instead as a weary, introverted veteran of his 40s, the kind of role I'm sure would have suited McQueen fine at that point in time. When the script was rewritten again by Adam Kennedy this template for Pitt, totally at variance with the original Cussler version, remained. The earlier script drafts did try to make use of the hurricane element and higher tension with the Soviets at the climax. It's now apparent I think that so much money was blown in the F/X for the underwater searching and the raising that they didn't have any left to craft an effective action sequence during the hurricane. Remember also that what *was* shot, and not used in the final cut of the film was a lengthy 1912 prologue sequence of the sinking and the Brewster character locking himself in the vault (the F/X sequence of the sinking was ultimately seen in a 1982 episode of the TV series "Voyagers" which is available on DVD BTW) so that should also tell you how much more was being spent at the expense of some items that would have helped the final end product (also cut from the film incidentally was a final scene of Anne Archer talking with Pitt aboard the Titanic after it arrives in New York. A publicity still for this exists and its clear this was meant to tie up the loose end of her relationship with Seagram which in the film just comes to a dead stop).
So ultimately the film was jinxing itself with a wrong approach to the script in many ways, blowing the budget on areas it could have scaled back on a bit to save room for other areas, and I think hiring a TV director in Jerry Jameson (who seems to have gotten the job only because he did "Airport '77") who wasn't particularly skilled at handling the actors well didn't help either. Far too many of the dialogue scenes that remain just fall flat or are poorly paced. And with all that said, I will grant the film succeeds on two levels. First, John Barry's beautiful score which remains the best one I think he ever did for a non-Bond movie, and the F/X of the finding of the Titanic and the raising of the ship still hold up as well. In fact, there is a moment when the wreck is found and they do a trail shot across the bow with the anchor chains that is eerily identical to the most famous shots of the wreck when it was first found. And seeing the ship come up was for many of us prior to 1985 a fantasy we liked to indulge in and its fortunate the film was made when we could still dream about it as by 1985 that was ended for all time.