When the Environmental Protection Agency was asked to review Interior Secretary James G. Watt's aggressive offshore oil and gas leasing proposal, it responded enthusiastically, calling the plan "suitably designed to protect the quality of the human, marine and coastal environments."

But EPA's official letter was dramatically different from the final draft prepared by the agency's technical staff.

That draft, based on research by EPA hydrologists, biologists and technicians who monitor offshore leasing activities, raised concerns about possible oil spills and water pollution. It also questioned whether the fast pace of the five-year leasing plan would allow adequate time to assess the environmental impact of the drilling activities.

The letter changed shape in the office of Paul C. Cahill, director of EPA's Office of Federal Activities. When the official response went out April 19 over his signature, the technicians' concerns were not included.

The transformation of the letter is not unusual, according to several EPA sources, who claim that technical experts are often overruled if their recommendations conflict with official Reagan administration positions. But an EPA source who had seen both letters called them "the most blatant example" of this pattern.

Watt's proposal would make 1 billion acres of offshore waters, the entire Outer Continental Shelf, available for oil and gas development in the next five years. It was released for public comment in the spring, and EPA was asked to review it as chief enforcer of air and water quality standards. The agency also has responsibility for monitoring discharges into the ocean during drilling.

Barring a court injunction, the plan will go into effect next month with the offering of several million acres off Alaska and California, and several Atlantic Ocean sites.

The plan, a vast expansion of the last administration's 60 million-acre offshore drilling proposal, has come under attack from several coastal state officials as an environmental threat for some of the same reasons cited in the EPA draft.

But the official EPA letter echoes the Reagan administration's defense of the plan as safe and a needed boost to domestic energy development.

The discrepancies between EPA's official comment and the draft prepared by its technical staff gave new ammunition to critics of the Watt plan, which was recently buffeted by a report by the General Accounting Office saying the plan will not generate the $18 billion originally forecast.

"The letter that EPA finally sent to Interior didn't talk about the problems this plan will really create," said Rep. Edward J. Markey (D-Mass.), a Watt critic who obtained copies of the two letters.

EPA's draft letter cited "major concerns" over the proposal to offer more offshore acreage for oil and gas development, while cutting the lead time for studying potential environmental impacts of drilling. The new schedule "will drastically reduce the ability of both the Interior Department and EPA to efficiently focus prelease environmental assessment activities on tracts most likely to be leased," the draft said.

The official letter noted that this "might be seen as a problem," but said EPA could complete "sufficient information gathering and exchange" through close coordination with Interior.

When asked about the failure of the official letter to mention the time problem described in the draft, Lou Cordia, special assistant to Cahill, said, "That the lack of time was a proposal in the draft. However, that is not the case."

Cordia also said that two or three drafts were prepared, but EPA sources said the draft obtained by Markey was circulated in the agency as the final version sent to Cahill.

The draft proposes that Interior reduce the number of underwater acres to be leased as a way of narrowing the area that EPA must study. The Cahill letter dismissed this idea as "contrary" to the directive of the Outer Continental Shelf Lands Act to speed domestic energy development. "All federal agencies must be supportive in achieving this goal," the Cahill letter said.

The draft also said that Watt's proposal underestimated the potential for oil spills. But the Cahill letter, while acknowledging that oil spills are "statistically probable," concluded: "To the contrary, a large find may occasion increased care in the field with consequential benefits to the environment." Asked if that statement was based on any statistical evidence, Cordia replied, "No, on a professional assessment."