President Carter said today he would "guess" that before the year 2000 a "larger, wider, deeper" Panama Canal without the multiple locks of the present one "might be in the interest of our national security militarily as well as economically."

The President used an energy-related news conference here to elaborate a bit on Thursday's mention at a Yazoo City, Miss., town meeting of a possible need for a new Panama Canal.

He spoke after helicoptering back with energy adviser James R. Schlesinger and Commerce Secretary Juanita Kreps from an hour aboard a 138 million offshore oil drilling platform 88 miles south of here in the Gulf of Mexico.

Carter said he has not explored in depth the question of a new canal and his remarks were conjectural. The administration, he said, is reassessing a report for President Johnson on the project.

At the time of that study, he said, "we did not have the additional problem then of very serious disputes with Panama on continued management of the canal under the 1903 treaty. We also did not have the additional problem of having to distribute Alaskan oil and gas to the eastern part of our country.

"I think at that time that price was considered to be shocking and unreasonable."

Carter noted private industry has just spent $8 billion on the Alaskan oil pipeline, and a pipeline costing possibly $12 billion is under consideration to carry Alaskan natural gas.

"So a new sea-level canal would not be unreasonable . . ." he said.

Carter also noted that "our present major warships, large tankers and cargo ships cannot presently use the Panama Canal at all . . .

Although Carter referred several times to "the report that was prepared when President Johnson was in office," he was apparently referring to a $22 million study ordered by Johnson in 1964 but completed in 1970 under President Nixon.

It reported that the cost of building a sea-level canal would be about $2.88 billion at 1970 price levels. Canal Zone Gov. Harold Parfitt told a Senate subcommittee today that inflation has raised this to $5.3 billion.

Sen. Mike Gravel (D-Alaska), who has been seeking a $7 million update of a 1970 study on the subject presented to President Nixon, said in Washington today that Secretary of State Cyrus R. Vance is "very enthusiastic" about the canal idea.

At the Senate hearing, Parfitt and Lt. Gen. Dennis McAuliffe, the senior U.S. military commander in the zone, said they do not anticipate a Communist takeover of the Panama government that could lead to a major crisis over the existing waterway in the near future. "The Communist threat is not a real threat in Panama," McAuliffe said.

Sens. William Scott (R-Va.) and Jesse Helms (R-S.C.), however, expressed concern about instability and Communist influence in the Panama government. And retired Adm. Thomas H. Moorer, former chairman of the Joint Chiefs of Staff, said any decision that would turn the canal over to a leftist government allied with Cuba could allow Soviet Russia to control it "by proxy."

The President flew here today after an overnight stay in Yazoo City, Miss., where the second time in his administration he fielded questions for 90 minutes from local residents and appeared to enjoy it thoroughly.

White House press secretary Jody Powell said Carter wanted to visit an oil drilling to show that - contrary to oil companies' newspaper advertisements - the administration is concerned about the need for new energy production, and not just about energy conservation.

The President was clearly fascinated by his visit to the six-month-old Zapata Yorktown, which drills for oil with equipment so modern that it can sink an angled hole 25,000 feet while bobbing in waves 30 to 40 feet high in water up to 2,000 feet deep, and never break a drill pipeL

Carter also said he was impressed with ". . . the obvious dedication of those who are searching for oil and gas off our shores to prevent the recurrence of environmental damage that occured in Santa Barbara . . . and recently in the North Sea." There were major oil spills in both places.

He said he hopes the nation can continue "with an aggressive exploration policy on the Atlantic seaboard as has been the case in the Gulf area."