NOW THAT “WATERWORLD” has finally washed ashore, there is much conjecture about the future of the $100 million movie — the megapic, in Variety argot. The Los Angeles Times last week ran a page one story bearing the headline, “Sea Epic’s Costs May Bring Wave of Caution”; yet only one day earlier a business page story in the Times had argued just the opposite notion — that Hollywood will go right on making big movies, whatever the fate of MCA’s waterlogged epic. Talk to the key decisionmakers at the studios, in fact, and you quickly realize there’s no way the megapic will become extinct. The studios need that big summer blockbuster, and they’re willing to spend whatever it takes to synthesize it. Moreover, the overseas market has given new importance to action pictures and special effects extravaganzas. At the same time, the “Waterworld” ordeal has underscored certain rules of survival in the megapic business — rules that the studios will be all the more meticulous about obeying. 1) Avoid post-apocalyptic settings. The apocalypse itself is good showbiz, but the “after” period tends to be rather downbeat and darkly allegorical. Denizens of “Waterworld” will kill for a scrawny tomato plant but, for some reason, everyone has the money to chain smoke and buy jet skis. Also, post-apocalyptic sets prompt set directors to conjure up Rube Goldbergian objects that are as expensive as they are unwieldy.2) STAY AWAY FROM WATER. For generations, studio production mavens have avoided ocean-bound shoots like the plague. It’s one thing to make some sappy romance like “The Blue Lagoon,” with the characters skinny dipping in a harbor, but the axiom has always been, “If it’s a big action piece, shoot it in a tank.” For 166 days, the crew of “Waterworld” struggled through fickle weather and rough seas only to underscore the wisdom of that dictum. 3) Never make an expensive movie that’s dependent on a star’s “vision.” Kevin Costner became enamored of the notion that action movies had become tired; his “vision” was to reinvent the genre in an entirely new frame of reference. The problem is that a megapic is basically a product to be manufactured, not a “vision” to be realized. Megapics need tough, disciplined producers and tight scripts, not visionary superstars. 4 ) Never start an expensive film when the director and the star are joined at the hip. In order to control a film, the studio needs to have at least one ally it can manipulate. A loyal director can instruct the star to stop rewriting the script and get to work. The two Kevins — Costner and Reynolds — had been united on three previous films, and they weren’t going to follow that drill. 5) Never make an expensive movie when the star and the director end up as bitter enemies. These bosom buddies were at each other’s throats at the end of “Robin Hood: Prince of Thieves,” and sure enough, such was the case at the editing stage of “Waterworld.” The two Kevins, as Reynolds now puts it, “had a difference of opinion as to the focus of the story.” He adds, “Maybe we should have kept our differences to ourselves, but I will not be made a scapegoat.” Costner took control in the cutting room, Reynolds went home to Seattle and declined even to attend the premiere. “I’ll see it at some point,” he told Variety’s Beth Laski. The version that finally emerged from this unusual process was uniquely “choppy”– in several cases, mismatched scenes seem to be heading first in one direction, then unexpectedly shift to another. 6) Big action pieces are becoming more effects-driven than star-driven — witness the surprise success of “Stargate.” In the case of “Waterworld,” MCA spent well over $ 30 million on its above-the-line players, including Costner, Dennis Hopper, Reynolds, plus the myriad contributors to the screenplay. In the world of megapics, that’s a lot of baggage to carry. 7) ONCE YOU’VE COMPLETED A MEGAPIC, find some way of writing the damned thing off. MCA performed this task brilliantly. Of the $175 million production budget, all but $12 million disappeared into a fiscal twilight zone when Seagram acquired MCA from Matsushita. Sony, it will be remembered, tried to perform a similar feat with “Last Action Hero,” but it ran out of prospective buyers. With the escalation of production costs, the real stars of future megapics will be those accountants who manage to “invent” some corporate shell to finance $100 million projects; the shell can then be dissolved or sold off. Since the Pentagon has learned how to make billions of dollars disappear into thin air, Hollywood should be able to follow suit. The conclusion of all this: There will be megapics in the future, indeed more of them than ever and at even higher prices, but studios will be more careful than MCA about observing the rules of the game. In “Waterworld,” Kevin Costner plays a mutant fish who, when times get tough, can simply take a dive. His financial partners have no such alternative.