In the light of the brightest stars I've ever seen, the supposedly deserted island of Espiritu Santo rose out of the Sea of Cortez off our starboard bow.

Then the anchor splashed into Ensenada Grande Bay, breaking the midnight calm and quiet of the Mexican waters. The subtle sound of jumping fish and the vague light outlines of a sandy shore in the distance seemed an incredible contrast to where I'd been only 12 hours earlier-weaving in and out of heavy, honking traffic along Los Angeles' Santa Ana freeway on a smoggy, humid morning.

There were no smog and certainly no horns here. But I was still warm, hot even. And it was more humid with scarcely a breeze, making it too uncomfortable to sleep in the small cabin just below where I now stood looking over the water. This was the first of what was advertised to be a different and exotic new anchorage every night during a week of cruising on Mexico's storied Sea of Cortez.

I had spent most of the 1960s as a newspaper correspondent poking into some of the more desolate regions of Latin America-from a trip through the Straits of Magellan between Argentina and Chile in a small boat loaded with wool and stone-faced shepherds, to a voyage across the world's highest navigable body of water, Bolivia's 12,500-foot Lake Titicaca.

But those are congested, peopled waterways compared to what I'd heard of the Gulf of California, as the Sea of Cortex is more commonly known to us gringos.

"Too many people are why I stopped running raft trips in the Grand Canyon," tour organizer Tim Means explained to me in La Paz. "Here I've gone a week or more without seeing anyone else except maybe a fisherman on the horizon. It hasn't changed much in 200 years."

Which is mainly why I and 17 others each had paid Means and his small San Diego company, Baja Expeditions, Inc., $695 for this island-hopping week between La Paz, near the tip of Baja California, and Loreto, 120 miles northward. Between these population centers there are no other towns and even the paved road is almost inaccessible, except by muleback, from the coast.

Our journey had started in mid-morning when Baja Expeditions (P.O. Box 3725, San Diego, Calif. 92103) bused us from San Diego to the Tijuana airport. We boarded an Aeromexico DC-9 and 90 minutes later were in La Paz, where taxis took us to the port and our boat. In the party were two avid fishermen and their shell-collecting wives; three scuba divers with underwater photography gear, and eight snorkeler-hiker-swimmers. Hometowns ranged from Napa, Calif., to Chicago, and Venice, Fla., to Reno, Nev. Ages were from 8 to 68.

The daily itinerary was to visit a different island bay or bays, anchoring throughout the day for diving, fishing, exploring and hiking. The boat would move only in the evening or night to a new location for the next day.

Our transport was the Baja Explorer, a 120-foot converted oceanographic research vessel with eight passenger cabins sleeping 23. There is a spacious dining-bar area and full diving facilities. A fresh-water maker, to desalinate 400 gallons daily and a dark room will be installed soon.

A second boat, designed by Means, is being built here. It is an 80-foot charter boat that will sleep 20 and be used when scuba equipment is not needed on camping and general expedition trips.

I spent most sweltering nights atop a thin mattress on deck. Temperatures do not cool until late October (air temperature was in the 90s, and the water was more than 80 degrees during my trip).

At dawn the first day I saw a dozen Mexican shark fishermen ashore in a makeshift camp. My disappointment at seeing people on what was to be a deserted island was overcome by curiosity and fascination with their activity, but for the next four days we saw no one else except men at great distances in 20-foot "pangas" or fishing skiffs.

It soon became apparent that as vibrant and lovely as the waters are here, the coast and islands are desolate, dry and austere. The region's stark beauty, like Arizona or Death Valley, is in its rugged, pristine landscape where greenery is confined almost solely to a few palms, mesquite and cactus.

Beneath the water, however, just feet off shore, is another world alive with trillions of brightly colored tropical fish and sportive, antic sea lions. The Sea of Cortez is reputed to have as many kinds of tropical fish as anywhere on earth, and the crystal clear blue and green waters made watching and photographing them easy and enjoyable. Snorkeling, I saw countless metalic purple damsel fish, trgger fish, golden groupers, spiny pufferfish, yellow striped sergeant majors, parrot fish, cabrilla and needlefish, all composed of more dazzling colors than a fourth-grade mural. Underwater visibility was often 100 feet and averaged 70.

Each day produced unexpected attractions: a special sunset, a walk through a lovely and canyon, pilot whales surfacing near the boat. The secon day, at Isla San Jose, 16 miles north of Espiritu Santo, several of us took a pre-breakfast trip by skiff through extensive mangrove swamps, home for countless snow egrets, great blue heron, belted kingfishers, pelicans, yellow and black crown herons and a variety of shore birds.

Later we watched scores of bottle nosed and Pacific white-sided dolphin skim through the water to swim and roll playfully in the boat's bow make for nearly 30 minutes. Almost nightly, Means presented informative slide shows on naturalist subjects in the area.

One day I made a trip at sunset with Means and two others to an extremely primitive Indian fishing village. We were welcomed by the female inhabitants into their palm-thatched huts. Turkeys, dogs, cats, mules, pigs and goats roamed the dirt yards where dates from nearby palms were being dried.

The men of these tiny isolated "ranchos" are fishermen. While they journey miles out to sea, the women rarely leave home. One old woman in this village, which is known locally as Carisalito, told me she had never been to La Paz, a city of more than 100,000,70 miles to the south, and only once to Loreto, a town of 4,000 that dates from 1697 and is 30 miles northward. Her trip was made by muleback two days to the highway, where she took a bus the rest of the way. Though Means had been a frequent visitor to the tiny community, the residents said almost no other Americans had ever visited there.

Starting in 1979, such myriad mysteries of the exotic Sea of Cortez will be unravelled for an increasing number of adventurous American travelers. The imaginative Means, 33, plans 36 trips next year and with the certainty of the two new boats hopes to simplify planning.

Means has taken more than 3,000 persons on his Mexico excursions since leaving the Colorado River rafting business five years ago. Group clients include numerous university charters from Pennsylvania to California, the Sierra Club, cetacean clubs and diving and photography groups, as well as the "open" tours such as mine.

The trips will include camping for a week on an island among sea lions and bird rookeries, and swimming among 700 varieties of tropical fish; whale-watching from rafts; camping along jungle beaches near Puerto Vallarta on Mexico's west coast, and exploring-sometimes for the first time-1,000-year-old Indian cave paintings (first found and publicized by Earle Stanley Gardner) that are deep in the desolate mountains of the central Baja California peninsula.

Trip prices, including air fare and meals, range from $695 to $495 for the cave painting excursions. They are scheduled every month except August and September because of summer heat and possible storms.

Means refers to the Sea of Cortez as "one of the most captivating, primitive yet accessible places on earth. It's the last frontier for tropical sea cruising and exploring in North America."

Such pristine solitude was brought home to me memorably the evening we left the tiny ranch of Carisalito and headed back to sea in our skiff. Ashore, by the day's last light, stood the woman to whom I had talked for the past hour. She had taken a palm branch "broom" and was industriously whisking away my footprints from her dirt yard.