After months of debate with coastal state officials and Congress, Interior Secretary James G. Watt yesterday made final his controversial plan to offer 1 billion acres of offshore waters for oil and gas exploration in the next five years.

The plan, largely unchanged from Watt's proposal last May, was immediately praised by oil industry spokesmen as a boon to domestic energy development. But it was denounced as an environmental threat by several coastal state officials and conservation groups, who said they plan to challenge it in court.

In a prepared statement, Watt said the accelerated development plan will "enhance the national security, provide jobs, and protect the environment while making America less dependent on foreign oil sources." Two-thirds of the nation's untapped oil lies under coastal waters, Watt said.

The plan calls for offering all of the Outer Continental Shelf for leasing to the energy industry in 41 sales over the next five years--more than a thousand times the area offered by the Carter administration. Watt stressed that he expects only a small portion of those offshore waters to be leased, but predicted that the program will speed up energy production.

Watt said the revised plan contains "stringent" environmental safeguards and takes into account the concerns of coastal state officials. It allows for minor delays in three scheduled sales of exploration rights, including one off the coast of New England that has come under fire from Gov. Edward King and state coastal zone managers.

However, officials in California, Massachusetts and Alaska--the states leading the opposition to the accelerated plan--said Watt failed to address their fears of environmental damage. They complained that the vast expansion of acreage being offered for leasing, combined with the fast pace of the sales, does not give them time to study potential impacts on their coastlines.

Alaska Gov. Jay Hammond said he plans to sue to try to delay several of the sales scheduled off his state's coast, and a spokesman for Gov. Edmund G. (Jerry) Brown said California may also take Watt's plan to court.

"The potential impacts to Alaskans are so severe that we have to use all possible means to protect the state's interest," Hammond said yesterday. Alaskan waters will carry the heaviest load of the offshore leasing program, accounting for 16 of the 41 sales of exploration rights. The area being offered there is more than three times the size of Texas, a Hammond aide said.

Hammond noted that most of the planned sales off his state's coast are located in "frontier" areas--those never before explored by the oil industry. He contended that technology is not advanced enough to protect against oil spills there that could damage valuable fish beds and coastal zones.

Energy industry officials said yesterday, however, that they are prepared to shoulder the leasing program, which the American Petroleum Institute hailed as "the most progressive ever adopted in the 28-year history of federal leasing of the Outer Continental Shelf." Some oil company spokesmen originally criticized the plan as too ambitious, but have since become supportive.

Under the Carter administration, offshore tracts of only about 2 million acres each were made available for leasing. The Watt plan calls for offering extensive "planning areas" of up to 133 million acres at each sale, allowing industry more latitude in deciding which areas will be developed.

Last week, Sen. Edward M. Kennedy (D-Mass.) and 28 other House and Senate Democrats signed a letter to Watt, contending that the plan undermines laws designed to protect the environment and to "assure efficient development of our natural resources." Watt responded with a letter yesterday defending the plan as environmentally safe and strategically essential.

"America must be prepared," Watt wrote. "It is much easier to explain to the American people why we have oil rigs off our coasts than it would be to explain to the mothers and fathers of this land why their sons are fighting on the sands of the Middle East as might be required if the policies of our critics were to be pursued."