Sometimes, dread takes physical form. I've wondered what it would be like to see a great white shark in the same water with me. After 30 years of surfing, diving, swimming and sea-kayaking coastal waters from Mendocino to Monterey, it finally happened.

During our last big northwest swell, two weeks ago, I was out for a dawn patrol at the Patch -- that shelf reef just north of the Bolinas Lagoon. After an hour riding the outside break, I saw an immense fin break the surface just within the outermost surf line, about 80 yards away."Wow. Pretty big seal fin," I thought. "Naw. Couldn't be that. Porpoise? Nope ..."

Another big wave swept past. That fin reappeared, this time attached to a gigantic, blue-black back, heading my way.

"Holy crap!" I yelped. "It's the man!"

Intellectual curiosity suggested I paddle closer, score a better look. But my body spontaneously vetoed any such move. Instead, primal, aversive instincts kicked in and I caught the very next wave, rode it in, then paddled south to surf the sandbar break at the lagoon mouth.

I didn't want to completely abandon one of the season's last good swells. And I guessed a fish that huge would shun those shallower, more chaotic waters.

But that encounter reminded me, there's a story I've been wanting to write. Can humans seeking recreation also take any actions to enhance their safety around waters of the northeast Pacific? Other than remaining on shore, I mean.

So, I checked in with two of the sharkiest people I know: John McCosker, of the Steinhart Aquarium at the California Academy of Sciences; and Sean van Sommeran, founder and director of Santa Cruz's Pelagic Shark Research Foundation.

McCosker and van Sommeran agree that the increasing numbers of unprovoked great white shark attacks worldwide is primarily due to many, many more people intruding on shark habitat.

But the percentage of white shark attacks which prove fatal has plummeted, probably due to the promptness and efficiency of modern medical care.

Which brings us to our first, and strongest recommendation: Always surf or dive with a buddy. If you do happen to get bitten, a pal can help you reach shore, can summon help, put direct pressure on a wound or apply a tourniquet.

"In Chile, where they typically dive alone, three of the six attacks since 1963 proved fatal," McCosker said. "Whereas, in California and Oregon, since 1950, only nine of 93 attacks were fatal."

The site you pick for your marine recreation also shades your odds. Statistics do bear out the notion of a "red triangle" off our coast. By far, most attacks since 1950 have occurred: off Sonoma County (nine); Marin (12); San Francisco, including the Farallones (seven); San Mateo (10); Santa Cruz (four); Monterey (nine); and San Luis Obispo (six). In counties north and south of this zone, attacks trail off to between one and four strikes. Please note, this is over the past half-century. Considering that millions of people have immersed themselves in these waters over that time, your odds of being bitten start off dramatically low.

To lower the odds still further, avoid hot spots (usually near seal haul- outs or basking areas) within the red triangle: Aņo Nuevo Island, the Farallon Islands, Tomales Point and Bird Rock.

Once in the water, avoid behavior that makes you look like a seal on the surface. This is a lot easier for scuba divers, who've learned to fin along near the bottom, and dawdle less during ascent or descent. Abalone divers and surfers are more limited, but can still pick their spots and times. The International Shark Attack Foundation, based in Florida, says most of the globe's great white attacks occur between 10 a.m. and 6 p.m. Dawn patrol turns out to be an excellent notion.

Of all senses sharks employ to locate prey, their strongest is "smell" (which in water, equals taste). So, a spear hunter should aim to get a stringer of bloody fish out of the sea, pronto. That also means it may be smarter to pee on land, instead of in your wetsuit. Urine helps sharks locate seal rookeries.

The next-strongest shark senses are vibration sensors in ears and lateral lines; these may function up to a half-mile away. Then the eyes, which are more sensitive to light than human eyes by a factor of 10. And finally, electrical receptors on the sharks' snout identify living beings and orient shark's jaws in the final moments before a bite.

Great whites have three basic attack modes. A "bump and nibble" approach seeks more information about an object. The "bite and spit" attack inflicts a serious wound, then the shark retreats to observe its result. Most devastating is the "breach and crush," where a shark leaps out of the water and pounces upon its prey.

"When it hits, it sounds like somebody smashing up a wicker chair," van Sommeran says.

That last form rarely occurs on humans. Were it common, our survival chances would be scant.

McCosker and van Sommeran politely disagree on bite and spit attacks. McCosker says it's a shark's preferred mode, and always is fully predatory. But van Sommeran says bites like that can be exploratory -- and a shark will break off an attack after it gets a mouthful of neoprene and fiberglass instead of tasty seal fat.

"Once sharks determine something is prey, all attacks I've seen are continuous and unrelenting," van Sommeran says.

They do agree, when a person is bitten, it's best not to try to yank your limb from a shark's closed mouth. That only adds to the deep lacerations. At some point, the shark will release the bite on its own.

If you do get a chance to strike back, don't bother beating on its snout. A shark's nose receives severe punishment from seals and sea lions all the time. Apparently, they aren't bothered much by it. Instead, aim any possible blows or kicks at the shark's eyes.

"Eyes are mission-critical gear for a shark. He'll seek to protect them," van Sommeran says.

"A shark that can't see is out of business," McCosker confirms.

At Bolinas, after surfing the bar for an hour, I tentatively headed back to the Patch. Jason, a longboarder with a close-cropped head and a long goatee, floated there. I told him about spotting a three foot-high dorsal fin earlier. He shrugged.

"Well, if you don't see them, it doesn't mean they're not here, either," Jason said.

"So. The dice we roll don't have all that many dots, eh?"

He laughed. "Yep!"

E-mail Paul McHugh at pmchugh@sfchronicle.com.

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