Either was possible, 30 feet below the surface of the Sea of Cortez, which separates the Baja California peninsula from the Mexican mainland. But the shape and high velocity of the apparition were strange. I couldn’t place it until another one appeared, then another, and soon more than a dozen, twisting and turning around us seven divers, coming eye-to-eye close before speeding away: sea lions.

describe the imageI should have figured it out sooner; I knew we were diving near a colony of the pinnipeds. But while I’d seen any number of sea lions above the water line, lolling in the sun or awkwardly dragging their blubbery bodies from rock to rock, I hadn’t imagined them transformed into these svelte underwater missiles, each one larger, stronger and faster than a human.

I was scuba diving in Mexico in the Cabo Pulmo National Marine Park, a 27.5-square-mile ecosystem with an unusual history and an uncertain future. At least 226 fish species live in the park, and it is home to the only living hard coral reef in the Sea of Cortez. But environmentalists fear that a major resort development could significantly alter this delicate fringe of Baja, both above ground and underwater.

Though I had been visiting the region for 15 years, I’d never been scuba diving in this spot. I was finally motivated to take the plunge because of the threat of impending damage. Today Cabo Pulmo remains hard to reach and full of sea life, but in a few years, the reverse could be true.

Underwater, a current carried us past the sea lion colony, and as we began to ascend, sunbeams lighted up schools of tropical fish: sergeant majors in their jailhouse stripes, Moorish idols trailing scimitar fins, giant hawkfish covered with squiggly Keith Haring lines. Back on the surface, a green turtle, one of an endangered species, circled our small motorboat.

After diving I retreated to the village of Cabo Pulmo, another unusual ecosystem. Perched on the edge of the reef, this dot on the Tropic of Cancer lies just 60 miles from the megaresorts of Los Cabos, but feels like a world apart. It is accessible only via a dirt road that runs along the southeastern shore of the peninsula, where you’re as likely to encounter cows and rattlesnakes as other cars. When I asked for a key to my bungalow at the Cabo Pulmo Beach Resort, I was told they didn’t use them.

baja mexicoThe town has perhaps 250 residents, including locals from one-time fishing and ranching families as well as refugees from Canada, the United States and mainland Mexico who spend some of the year here. Even as nearby parts of the peninsula have changed exponentially over the last decade, bringing big-box stores and an expanding network of paved roads, power lines and cellphone towers, Cabo Pulmo has remained sleepily off the grid. Solar power and generators provide electricity. Water comes from a well or is trucked in. I counted one grocery store, five restaurants and three dive shops that were regularly open.

Also known as the Gulf of California, the Sea of Cortez is a 62,000-square-mile finger of water with a distinguished list of admirers. The ocean explorer Jacques Cousteau called it “the world’s aquarium.” In 2005, Unesco declared 244 islands and coastal areas in the Sea of Cortez, including Cabo Pulmo, a World Heritage Site. In 1940 John Steinbeck traveled the length of the sea with the marine biologist Edward F. Ricketts and penned “The Log From the Sea of Cortez,” a blend of science, philosophy and travelogue. Of Cabo Pulmo Steinbeck wrote:

“Clinging to the coral, growing on it, burrowing into it, was a teeming fauna. Every piece of the soft material broken off skittered and pulsed with life — little crabs and worms and snails. One small piece of coral might conceal thirty or forty species, and the colors on the reef were electric. The sharp-spined urchins gave us trouble immediately, for several of us, on putting our feet down injudiciously, drove the spines into our toes.”

In the ’70s and ’80s, though, overfishing decimated the region’s sea life. Local people lobbied to protect the area, and in 1995 the Mexican government made Cabo Pulmo a national park, banning fishing. This turned out to be a huge environmental success: between 1999 and 2009, the fish population grew by 463 percent, according to a study from the Scripps Institution of Oceanography — the biggest such increase ever measured in any marine reserve in the world. It was also an economic success, as one-time fishermen became dive, snorkel and kayak guides.

Consternation about the reef surfaced again in the late 2000s, when a Spanish developer announced plans to build a complex with 30,000 hotel rooms less than seven miles north of the park. The company’s vision for Cancún-style sprawl was the opposite of the low-key eco-tourism that has taken hold in Cabo Pulmo. Conservationists noted that construction runoff can kill coral reefs, as can too many clumsy divers. A coalition led by Mexican environmental organizations protested; President Felipe Calderón responded by canceling permits for the megaresort in June 2012.

And that was supposed to be that. Just two months later, though, another Spanish developer resurrected plans for a resort in the same spot, filing with Mexico’s Ministry of the Environment and Natural Resources to begin an environmental impact assessment. The project was called (with unintentional irony) “Los Pericúes,” after an indigenous group made extinct by Spanish colonization.

The plan includes two golf courses, a marina with 300 moorings, 3,450 hotel rooms and 6,650 residential units. Spread over 14.5 square miles, the project would take such precautions as erecting a desalination plant and building away from sand dunes to limit interfering with nesting sea turtles. The ministry issued a report saying that the project would only “marginally” affect the marine and terrestrial ecosystems, and that Los Pericúes “does not represent a significant change from the trend in the absence of the project.”

The organizations Wildcoast and the Mexican Center for Environmental Law have said they will continue to put up a fight. Not all environmentalists think the project would be a disaster, though.

Patrick Moore, a founding member of Greenpeace, is a part-time resident of Cabo Pulmo, having bought property here in 1999. He said he believed that a big resort could, in theory, be done right.

“I think most people would accept a sustainable development,” he said, adding, “It would bring economic benefits.”

Some locals are not so sure.

“What they’re doing could put a lot more pressure on the reef,” said Cole Barrymore, an owner of Cabo Pulmo Beach Resort, one of the oldest hotels in town. “If the large resort goes ahead and the national park doesn’t enforce new rules on numbers of divers, it’s not going to be diving like it used to be.”

It may be, though, that nature itself keeps industrial-scale tourism at bay. After arriving in Cabo Pulmo I had to wait three days to dive because of the high winds that affect the area four months of every year — the same months that Mexico’s northern neighbors take their winter vacations. In the summer, the air is still and underwater visibility can extend to a spectacular 100 feet, but temperatures on land climb into the 90s.

Two days after diving with the sea lions I went out to a different part of the park. From the motorboat, I watched mother-and-child humpback whales breach the surface and slide back into the deep. After I tumbled off the gunwale and began my descent, a school of cownose rays, a near-threatened species, soared by overhead. Blue and black angelfish, yellow butterflyfish and speckled puffer fish patrolled their respective patches of coral, while a moray eel glared from beneath a rock. For now, at least, the reef was teeming.