Now it is all over but the launching. According to 20th Century-Fox, ''The Abyss'' cost $43 million. Executives at other studios think it may have cost more. Two less ambitious underwater efforts - ''Deep Star Six'' and ''Leviathan'' - have recently failed at the box office. ''The Abyss,'' which was originally to have opened July 4, was delayed for more than a month by production and special-effects problems, resulting in a late start on the summer. And Fox must teach a considerable portion of the American public what the word ''abyss'' means.

There were reports from South Carolina that Mr. Harris was so angered by the physical torment of the film and the autocratic ways of Mr. Cameron that he said he would refuse to help sell the picture. Mr. Harris denies that he has any animosity. Polite and soft-spoken, he showed up at Fox's press junket for the film last weekend. This is not to say that his memories lack pain. For weeks, he was encased in a suit where he appeared to be breathing fluid rather than air. Breathing the fluid, which filled his suit, would supposedly allow his character to descend into a bottomless pit in the floor of the Caribbean Sea. Such a fluid actually exists. It was tested once on a scientist - who nearly died -and over the last 20 years has been tested on numerous animals, which survived. In one of the movie's most uncomfortable scenes, a white rat is really locked in a box of the liquid fluorocarbon.

Movie stars being less expendable than rats, Mr. Harris held his breath inside a helmet full of liquid while he was being towed 30 feet below the surface of the tank. When he could no longer hold his breath, he pushed open his face plate and Al Giddings, a professional diver who was shadowing him, swam over to push a regulator attached to an oxygen tank into his mouth.

''The worst moments for me were being towed with fluid rushing up my nose and my eyes swelling up,'' says Mr. Harris. ''Once, the regulator was put in upside down so that one-half of what was going into my lungs was water. For a brief second, I thought, 'This is it,' before Al swam over and put the regulator in right. And then I was mad at myself for feeling that panic.'' He was not the only actor to feel panic. ''I had to play mind games with myself any time I had the helmet on,'' says Mary Elizabeth Mastrantonio, who plays Mr. Harris's estranged wife and the designer of Deepcore, the submersible oil rig on which Mr. Harris works. ''I can be as hardy as the next person but only for a limited period of time. I don't have the stamina. To keep from being panicked, I had to stare at the surface where the light was coming from.'' But what happened when there was no light? Michael Biehn is a strong swimmer, the winner of YMCA breaststroke races during the summers of his youth. ''But we were weighted down to the bottom of the tank,'' he says. ''There were weights on our ankles and waist. If something went wrong, we couldn't swim to safety.'' Mr. Biehn plays the closest thing the movie has to a villain, a Navy lieutenant who goes crazy with water-pressure-induced psychosis. ''Once, we lost all power,'' he says. ''It went completely black. It took 90 seconds of panic before I realized I was carrying a prop flashlight and turned it on and everybody gathered around the light.''

Despite the fact that James Cameron movies meld state-of-the-art hardware with relentless action and dangerous special effects in creepy environments, no one has ever been seriously injured on one of his films. On ''The Abyss,'' there were only the ear infections that are an occupational hazard for divers. And, when the smaller tank was overchlorinated, the hair of some of the divers was bleached or burned off.