The boat rose and fell relentlessly, one ocean swell after another. I leaned over its stern, fixing my eyes on the decoy floating a few yards away. I had watched it for hours, barely glancing at the elephant seals hauled out on the nearby island.

In my eagerness to see who might nibble on the bobbing object, I was reluctant to look away for even a moment.

I was on a day-long expedition to the Farallon Islands, about 50 kilometres west of San Francisco. The craggy islands are a familiar sight for Bay Area beachgoers on a clear day, but most people don't know that they support vast seabird colonies and mammals such as sea lions, dolphins, elephant seals and humpback, blue and gray whales.

The Farallones are also home to some of the largest great white sharks on the planet.

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My husband Andrew, our friend Neil and I joined a dozen biologists, volunteers and wildlife watchers for the trip - part of Sharktober, an annual Bay Area celebration of the great whites' return from their oceanic migration each September through November. Along with land-based educational events, the excursions immerse aspiring seafarers in the Farallon Islands' natural history and marine ecosystem.

Shark Stewards, a nonprofit group that promotes sound stewardship for sharks and all marine life, organises the events and tours. Its director, marine biologist David McGuire, led our expedition as one of several weekend day trips during Sharktober.

We boarded the Silver Fox, a chartered 50-foot fishing boat, at Fisherman's Wharf in San Francisco. Although thick November fog dulled the sunrise, the forecast was less volatile than usual. I still pulled on my burliest cold-weather clothing, including a waterproof jacket and pants.

The Farallones are fierce. Because of savage weather, perilous currents and ragged rocks, the Greater Farallones National Marine Sanctuary estimates that more than 400 shipwrecks might surround the isolated islands. Local Miwok referred to them as the "Islands of the Dead," and mariners called them the "Devil's Teeth." Compounding the sense of danger is the sharks' fearsome reputation, which only now is beginning to recover from decades of misperception and demonisation.

The Farallones are a marine sanctuary protected by a number of state and federal designations. As a result, they're a haven for at least 25 endangered and threatened species, more than a dozen nesting seabird species and 36 marine mammal species. And, of course, the sharks.

We puttered into San Francisco Bay. Fog and water congealed into an indistinct smudge, dampening visibility and sound. A container ship lurked in the mist, occasionally breaking the eerie stillness with a horn blast. We were already in another world.

As if to emphasise the transition, harbor porpoises leaped out of the water before we even saw the Golden Gate Bridge. Eventually, its pylons rose beside us and disappeared into the gloom. We passed Point Bonita lighthouse at the bay's entrance. Periscoping sea lions eyed the boat, and brown pelicans flew overhead.

The only human-inhabited island in the archipelago is Southeast Farallon Island, where two windblown dwellings house researchers. Photo / Erin E. Williams
The only human-inhabited island in the archipelago is Southeast Farallon Island, where two windblown dwellings house researchers. Photo / Erin E. Williams

We were surprisingly close to Marin County's Muir Beach when we saw our first humpback. It exhaled, and the breeze wafted a comically sour plume from its blowhole into our faces.

A tail rose from the water: a California gray whale heading south on one of the world's longest migrations. We turned west into the Gulf of the Farallones, riding against 3-metre swells through the clearing fog. I silently prayed to the motion-sickness patch stuck behind my ear.

We rolled up and down the swells and rubbernecked at whales for 2½ hours until we reached the Farallones' granite island ridge, once part of a Southern California mountain range. Ocean waves thundered into jagged, sky-scraping spires.

Around Southeast Farallon Island, thousands of common murres circled in the air. "This is North America's largest seabird colony," McGuire explained. "In springtime, more than 300,000 seabirds nest here."

The boat idled in a cove. Although the islands are off-limits to the public, two strikingly desolate houses sit below a lighthouse-topped summit. For years, hardy scientists have taken shifts researching and living on the island.

I smelled the elephant seals before I saw them squabbling with each other under the cliffs. They are one of the sharks' key food sources. Each fall, between 15 and 50 sharks arrive - most of them large and all of them hungry. They're part of a population of several hundred that scientists have identified from Mexico to British Columbia.

A couple of months later, fattened on elephant seals and sea lions, the sharks depart the Farallones. Some travel to the "white shark cafe" in the mid-Pacific, a gathering place where scientists suspect that they feed and breed. Others swim to Mexico. Their round-trip migration can approach 8000km.

On the deck, a graduate student named Nikki prepared a decoy: a child's stuffed black wet suit with dive fins extending from its legs. Although attracting sharks is forbidden in the protected waters, Shark Stewards holds a permit for the lure.

Nikki chucked it over the side of the boat, where it made a fair impersonation of an injured seal. But would a shark fall for the ruse?

"We need something that looks like a food source, since sharks aren't interested in humans," McGuire said. "That said, sharks are pretty smart. If they see it once, they probably won't investigate it twice."

During the trips, McGuire and his team employ a toaster-size, remotely operated vehicle to observe sharks underwater without disturbing them or altering their behavior. The team records and shares data - including with the public through educational outreach and as part of National Geographic's Open Explorer program.

"These are everyone's sanctuaries," he said. "If people don't experience them, it's harder to appreciate them and understand the importance of protection."

Although attracting sharks in the sanctuary is prohibited, Shark Stewards holds a permit for a seal decoy. Photo / Erin E. Williams
Although attracting sharks in the sanctuary is prohibited, Shark Stewards holds a permit for a seal decoy. Photo / Erin E. Williams

An intern dropped the little robot overboard. It scooted away and disappeared while McGuire operated it from a tablet.

The decoy was getting too close to the boat. I followed McGuire's request to pull its line as we turned, jerking it to mimic a panicked seal.

"Yeah, that's definitely going to attract a shark," said Neil, rolling his eyes.

The motion-sickness medication was losing its battle with the swells and the elephant seals' spectacular stink. Andrew and I felt seasick.

The three of us watched the decoy from the stern. Every once in a while, I fixed my eyes on the island to quell nausea.

We lingered while McGuire and his team operated the robot. Whales breached off the bow and seabird after seabird buzzed the boat. After a couple of hours, Andrew had to take a break. He disappeared into the cabin.

Some people say that we have an innate sixth sense when it comes to sharks, that our hair stands on end and our skin tingles. I had no such premonition. I didn't feel like a shark encounter was imminent. All I felt were nausea and dimming hope.

Maybe that's why I was dumbstruck when I heard a splash.

It was over before I understood what was happening. I turned to the sound. A gigantic shape broke the water just three metres from the boat.

A great white shark.

Clearly not fooled by the decoy, it bumped it with its nose and cruised by. The skin on its back was a beautiful smooth, liquid, metallic gray. After a few seconds, it slid back underwater.

Overcome with excitement, Neil and I shouted expletives while pointing at the disappearing eddies.

McGuire confirmed the sighting, and everyone cheered.

This was the moment that Andrew emerged from below deck. He had only been gone for a minute or two. "No," he said. "You didn't see a shark."

I nodded. Now it was his turn to curse.

I'd like to think that I saw the iconic shape of its dorsal fin slice through the water. It happened so quickly, it's tough to be certain. The shark appeared colossal to my inexperienced eye, but McGuire later confirmed that it was somewhat small compared with the Farallones' largest great whites.

The sun sets behind the Golden Gate bridge on the expedition's return trip into San Francisco Bay. Photo / Erin E. Williams
The sun sets behind the Golden Gate bridge on the expedition's return trip into San Francisco Bay. Photo / Erin E. Williams

"Most sharks here are at least 14 feet long, and they can reach 20 feet," he said. "It was maybe 12 feet."

Daylight was waning. It was time for a final errand: a search for a friend's camera. Commercial diver and documentary filmmaker Ron Elliott lost it when a shark bit him on the hand while he was filming a month before. This was the only bite Ron has suffered in 400 shark encounters during decades of diving the Farallones.

The remotely operated vehicle poked around in underwater nooks and crannies but did not locate the camera. We turned back to the coast.

We sat on the bow during the return trip. Hundreds of maroon sea nettle jellyfish drifted under the boat, and dolphin pods followed our wake. I lost track of the day's whale count after a dozen. Neil occasionally leaned across Andrew and asked me, "Do you remember when we saw that shark and Andrew missed it?"

My head bobbed up and down like a decoy.