The Interior Department, after four years of dry holes and disappointing exploration, has lowered its estimate of recoverable oil in the nation's offshore waters by more than half.

The new calculations, not publicized although they were completed more than three months ago, indicate that 55 percent less oil and 44 percent less natural gas are beneath the sea than was estimated in 1981.

In the so-called "frontier" areas off Alaska, where the massive Prudhoe Bay discovery once fueled hopes of almost limitless domestic supplies, the estimates dropped even more precipitously.

"What's happened in Alaska is very discouraging," Interior Secretary Donald Hodel said in an interview yesterday. Citing "unsuccessful drilling ventures in highly prospective areas," Interior has lowered its sights off Alaska by 73 percent for oil and 78 percent for natural gas.

The findings suggest that offshore energy production, which Interior officials predicted four years ago would supply as much as 40 percent of the nation's future oil needs and 30 percent of its natural gas, will not meet expectations despite the Reagan administration's aggressive leasing policies.

"This changes people's minds about the nation's energy future," said William D. Bettenberg, head of the department's Minerals Management Service. "I can't say that there's anything very encouraging."

Interior's new estimates were revealed yesterday in an Office of Technology Assessment report warning that the nation may be heading for more trouble as oil imports rise and domestic reserves dwindle.

"Oil imports, which have declined in recent years, are expected to gradually increase and may again reach the high levels of the 1970s," when imports peaked at more than 9 million barrels a day, the congressional agency said. It said imports of that magnitude "would make the country more vulnerable to supply interruptions and would increase the trade deficit."

The report was prepared for a House Merchant Marine and Fisheries subcommittee hearing today on federal leasing law, and a panel aide called it "a little bit earth-shattering."

The OTA researchers also said they were "astounded" by Interior's new estimates, forwarded last February when the report was still being prepared.

"When we were talking about revisions to offshore leasing law in 1973, industry was talking about these grand resources offshore," said James W. Curlin, who directed the OTA project. "The assumption was that our domestic resources were simply there for the taking. Now we had better start looking at the alternatives."

But industry representatives discounted the new calculations, contending that the department did not consider data from recent deep-water discoveries in the Gulf of Mexico and off California.

"If these figures were reflective, companies would not be acquiring leases and accelerating exploration," said Steve Chamberlin of the American Petroleum Institute.

He acknowledged, however, low industry interest in once-promising frontier areas. "That's because they haven't found anything yet," he said. "They are going where they can realize a return on their investment sooner -- the Gulf and California."

Although the findings appear certain to trigger new debate on energy policy, opinions were sharply divided yesterday over what the pessimistic estimates mean for the administration's controversial offshore leasing policy.

That program has been under attack since 1981, when then-Interior Secretary James G. Watt launched a campaign to put as much as 1 billion acres of the outer continental shelf under lease by 1987.

In view of Interior's new projections, "some people are going to say 'why bother?' " Hodel said. But he and other Interior officials contended that the findings underscore the need to speed development of offshore areas, especially those of known productivity.

"If there's oil out there, we need to know about it," Bettenberg said. "If not, we also need to know that."

Interior's new estimates are likely to increase pressure on California, where state officials' concern about environmental impact of unrestrained energy development has been largely successful in blocking large leases.

According to Interior's new analysis, "the greatest petroleum potential for the Pacific region" is off southern California. California is the site of the only major offshore oil field discovery since the government accelerated exploration in the wake of the oil embargoes of the 1970s.

The OTA report also recommended more intensive exploration, especially in the Arctic and deep-water areas. But it also noted that low oil prices and the high costs of extracting oil from such areas may require federal policy changes that "shift more of the economic risks to the government."