SAN CRISTOBEL Island materialized out of the equatorial haze at noon on our seventh day out of Panama, so we had all afternoon to worry about the consequences of our long-planned but illegal visit to the Galapagos.

Lying astride the equator 600 miles west of South America, the Galapagos are the only possible stop in the 4,000 miles from Panama to the Marquesas Islands in French Polynesia, the longest leg of our yacht Felicity's around-the-world cruise. The islands offer fresh fruit, vegatables and fuel, and the offer was irrestible.

The Galapagos are one of the world's most fascinating island groups. Far from any continent and cooled by the Humboldt Current, they teem with unique wildlife, tropical iguanas and sea turtles coexisting with penguins and fur seals.

Some species have adapted from jungle environments and overflow into the sea. The physical changes they have undergone helped inspire Charles Darwin's theory of evolution.

The waters abound with life. As we drifted through the doldrums, an equatorial band of light winds and calms that lies across the 750-mile course from Panama to the Galapagos, we were visited by pilot and sperm whales, porpoises, sharks, swordfish, sea turtles, seals and sea lions. Even a squid landed on Felicity's deck during a squall to join the nightly harvest of flying fish.

It seemed that the great creatures of the deep came to the doldrums on vacation to bask in the sun.

Overhead, a multitude of birds gracefully glided back and forth, looking for lesser prey. Great frigate and gulls albatrosses, boobies, petrels and gulls inspected us as we neared the islands. The birds provided a check on our navigation as they returned to their island roosts each evening.

The wildlife and birds of the Galaspagos have little fear of man because thed are rigidly protected by the Ecuadorian government, which has declared the islands a national park.

The same laws that protect the birds and animals threatened us with jail or a fine. Foreign pleasure yachts are prohibited from entering Galapagos waters without advance authorization by the government in Quito. We had tried to obtain a permit through the Ecuadorian ambassador before we left Washington last year. He did not reply to our letter. We telephoned the Ecuadorian embassy in Panama City after we transited the Panama Canal, but there was no answer.

Permit or no permit, we sailed from Balboa intending to stop at least briefly at Wreck Bay on San Cristobal. Our plan was to claim engine trouble, invoking the law of the sea that all nations must give refuge to vessels in trouble.

Happily, the naval commander at Wrect Bay was a friendly young officer, Lt. Milton Vallejo, who had visited Annapolis last year during the Bicentennial. He accepted our story (he probably has heard them all) and let us stay 48 hours for "repairs."

He agreed to sell us diesel fuel and let us to sail in tandem with his patrol boat to Floreana Island. The other islands - especially Santa Cruz where the Charles Darwin Research Station and park headquarters are located - were strictly off limits, he said.

We jumped at the chance to linger two more days, especially at Floreana. This volcanic island is the site of fabled Post Office Bay and the home of the Wittmers, a remarkable German family who settled in the Galapagos 45 years ago. Wolfgang Allendoerfer, a young German, was hitching a ride with us from Panama to Tahiti, and his presence apparently helped.

After buying fresh eggs, breads, fruits and lobsters by bargaining door-to-door among the villagers, we got under way at midnight to make our 10 a.m. rendezvous at Post Office Bay, 60 miles away. Along the way we wrote postcards and letters to be deposited in the oak barrel that serves as the "post office." From 1790 to 1860, whaling ships left mail in the barrell for homebound vessels to pick up, a sacred duty on the lonely sea.

Margaret Wittmer is postmistress as well as matriarch of Floreana. The post office is a corner of her family dining room, which also serves as a restaurant. She proudly stamps "Galapagos Barrel Mail" on outbound envelopes.

Mrs. Wittmer and her late husband Heinze left Germany in 1932 to join a handful of countrymen and start a new life on uninhabited Floreana. The others have long since died, disappeared, or departed - some mysteriously - but the Wittmers stayed and prospered. There are several Ecuadorian families now, some of whom work for the Wittmers.

Son Rolf runs the family's varied farming enterprises, but it's Margaret Wittmer, now 78, who rules the roost from her simple home beside a black sand beach a few miles from the actual bay.

Almost from their first days in the Galapagos, the Wittmers have been visited by yachtsmen bound for the South Pacific. When we arrived, Mrs. Wittmer offered us cold drinks and beer. Her daughter Inge brought out three worn scrapbooks with entries about each of the yachts that had called at Floreana during the last 20 years. Felicity was the first entry in three months. Before the government clamped down on visiting boats, Rolf Wittmer said, as many as 80 yachts were received each year.

That night the entire Wittmer clan pitched in to prepare a German dinner of roast duck with gravy, potatoes and asparagus for us and the crew of the Encantata, a local Ecuadorian charter yacht.

After dinner Rolf give us our only glimpse of the large land turtles for which the islands are named; he had five in his coral. One woke and waddled over to solicit a scratch under the chin.

Few of the turtles still roam free; most now are under the supervision of the national park service, either in captivity or in controlled herds. They were almost wiped out by the whalers and the domestic animals and rats the whalers brought to the islands.

The next day we saw more wildlife. Birds were everywhere; iguanas strolled casually about the tiny settlement and lazily sunned themselves; a fur seal swam out to Felicity and played without inflatable dinghy. The crystal clear water was alive with colorful fish.

As we prepared to leave, we paid Mrs. Wittmer a last visit. Once she asked her yachting visitors for only ammunition and books from the outside world. Today, cooking spices and radio batteries are hot items.In exchange for several boxes of curry powder and pepper, she gave us two pounds of fresh meat.

"You won't get this every day," she said with a twinkle in her eye.

"Go 100 miles south of here, and you'll pick up the tradewinds. From there on it's easy," she said. "It will take you between 21 and 28 days to reach the Marquesas Islands.

She was right on the money.