For the third straight day of his long Memorial Day weekend, President Carter spent most of his waking hours on the water, this time leaving at 2:45 a.m. for 11 hours of deep-sea sport fishing 45 miles off the Georgia coast.

"We had a good day," he said as the 35-foot boat No Name II, owned by two local businessmen, pulled back into the St. Simons Marina this afternoon. "We got a few kingfish, bonita and dolphin," Carter said. There appeared to be about a dozen fish in all.

Mrs. Carter who spent part of the time finishing her briefing books for the two-week tour of Latin America she starts Monday, said she caught the biggest one, a 20-pound king mackerel.

She said she was "almost" ready for her trip. Asked what other preprations she had to make, she said she had to pack her bags, and I'm going to talk to Jimmy tonight for about three hours. He's already promised me."

The day seemed ideal for fishing, sunny and hot with just enough breeze to be refreshing. As the President walked slowly by reporters waiting on the dock he talked about fishing.

But on other topics he said he has not decided whether to reappoint Arthur F. Burns as head of the Federal Reserve Board, that the strategic arms limitations negotiations between the United States and the Soviet Union "have been going rather well lately," and the two meetings planned by mid-September between Secretary of State Cyrus R. Vance and Soviet Foreign Minister Andrei Gromyko would "probably" be in Geneva.

The President said his daughter, Amy, who boarded the boat in the dark 11 hours earlier carrying a sweater and a box of cookies, had cooked some fish, "but they're too heavy for her to pull in. She starts them in. We give her credit for it."

Carter didn't say whether he caught any fish. The presidential party included Jerry Edwards and Jack Hart, owners of the No Name II, old fishing buddies Jim Bishop and Carlton Hicks, and White House physician Dr. William Lukash.

They went out almost to the gulf stream, and described the seas as "smooth . . . just beautiful."