Aboard a floating fortress of solitude in the Sea of Cortés

Up to catch the sunrise, cruise passengers nursing coffee watched as the Safari Endeavour glided past the Baja Peninsula's ragged coast. Rays played over the cliffs, and each thumb-shaped cove and crescent beach came into view for a minute or two, then slid out of sight, disappearing astern.

Fifty yards off the starboard bow, a whale surfaced to breathe, blowing an airy spray of mist and leaving a widening circle of ripples. On the port side, a squawking band of sea gulls hovered over a rocky islet shared by a colony of croaking sea lions.

They and the Endeavour were the only signs of life, or so it seemed to this first-time visitor to the Sea of ­Cortés, the 700-mile-long finger of ocean separating the Baja California Peninsula from the Mexican mainland. It's also known as the Sea of Cortez, Gulf of California and Vermilion Sea.

In nearly a week on the ship, touring at sea and on land, we'd spotted three of the seven whale species that frequent the region, part of the abundant stew of marine residents, including dolphins, green turtles, mobula rays and dozens of birds. We'd walked through a tiny fishing village. Hiked the narrow trails to dusty cliff-top ridges. Snorkeled in glass-clear coves. But except for a brief glimpse of two small sailboats, we'd had the lonely Sea of Cortés all to ourselves.

Behind us on the bridge, Capt. Jill Russell, the Endeavour's skipper, was in her element, peering through binoculars and reading weather signs in the clouds. With energy to burn and a hands-on approach to management, she checked the wind, wondering if the day's planned expedition should be canceled.

Buttonholed by a photographer who asked where the ship was ­heading, she was ready with a snappy comeback. "I don't know," she answered, pausing for comic effect. Then she grinned. "But I'll know when we get there."

A few of the passengers, mostly cruise veterans expecting an orderly progression of ports and tours, suddenly felt unmoored. "Now what?" asked a retired lawyer from Maryland. "Can they change the route just like that?"

The 86-passenger Endeavour, an Un-Cruise Adventures ship, is a solo act. Only Holland America and ­Carnival sail anywhere close by, and that with one ship each. A third cruise vessel, Lindblad Expedition's 62-passenger Sea Bird, also sails here in January and February, exploring Baja's virgin beaches. But only Un-Cruise Adventures goes off the grid, leading shore expeditions to deserted bays and coves, places like Ensenada Grande, Bahia Aqua Verde, Espiritu Santo and El Cardonal.

We'd expected to spend that day on shore, wandering along the beach, looking for shells and driftwood, snorkeling among the rocks, looking for angelfish, emerald wrasse, sea horses and even sea turtles. Some passengers planned to kayak across the bay; the Endeavour's techno-savvy, storage-and-launch rack (nicknamed the "pickle fork") lowers the 20 two-man kayaks to the water level for easy loading. Other passengers had signed up for the guided hike through the brush-and-cactus forest that looks like a ­desert but isn't.

"We'll be exploring what scientists call a "low elevation deciduously-treed jungle," promised naturalist and hike leader Paulino Perez during the previous evening's slideshow. But with choppy seas and heavy surf, beach landings were put on hold.

I stood by the rail instead, searching for leviathans. Sperm whales and gray whales, humpbacks and blues, even fin whales and giant whale sharks frolic in these protected blue waters, "the world's aquarium," according to marine biologist and diver Jacques Cousteau, who labeled the region the "Galapagos of North America."

Idling near the rocky pinnacles called Los Islotes we spotted pods of dolphins, sea lion colonies sunning, and brown pelicans and blue-footed boobies jockeying for sunny perches. But the week's highlight was the shore tour to Magdalena Bay, on Baja's Pacific Coast, to see the ­migrating gray whales.

The drive was long, two hours on a continuously winding road. But by midmorning 40-odd other travelers and I were seated in four large pangas [fishing boats] and motoring slowly across the lagoon. At first, the lagoon seemed empty, the whales gone. But just as we'd decided the trip was a bust, a mottled hump silently broke the surface nearby. Then a second shiny wet back emerged with a calf by her side.

As the day warmed, so did the whales, rolling sideways to inspect us with one eye and "spy hopping" straight up between the pangas for a 360-degree look-around. A few whales slid next to the panga, as if courting the touch of a human hand. Whales were everywhere, gently cruising, ­rising, breathing and diving.

Whaled out, we were ready for lunch by 1 o'clock, served at a local cafe that offered plates heaped with chicken enchiladas, tacos, rice, beans, chili verde, sliced mangoes and a fresh green salad, served with a soda or beer.

How much leeway does a ship captain have in a place like the Gulf, where the itinerary can be flexible, I wondered. Was the Magdalena Bay tour available on a day's notice?

"It's rarely a problem," said Russell. "That's what I like about out-of-the-way places like the Sea of Cortés. The big cruise ships depend on stopping at ports with facilities; they have to go where they're expected. But here in Baja, we get to decide when and where to anchor and what to do that day."

"The vessels were already right there, in Alaska, and they came on the market at the right price," said Un-Cruise spokeswoman Sarah Scoltock.

When the purchase was complete, the company's fleet of ships, now sailing under the Un-Cruise Adventures logo, had doubled. So, too, had the number of cruise itineraries, each designed with the idea that independent travelers want to be active participants. "Comment cards and feedback say that our passengers want good food, a choice of wines and the little luxuries that matter," she said. "But they don't want to sit in the lounge and look out the window. They want to make their own adventure."

In the lonely Sea of Cortés, that goes without saying.