IT'S MARCH IN the Bay Area, and summer has sprung. While the rest of the country catches its collective breath after an arctic cold snap, San Franciscans are (or were, at press time) making tentative plans for weekend games of Frisbee golf or trips to the beach. There's a good chance it will at least be balmy, if not bathing suit weather. I took that bet recently and headed to west Marin County to do a little touring – and eat some oysters.

I more or less broke even. As one resident of Point Reyes Station put it, "It's raining on one side of the street, sunny on the other, and the wind's blowing down the middle." My trip north from San Francisco to Point Reyes was like driving through a car wash. At the coast the sun cracked partway through the clouds and illuminated the whitecaps on Tomales Bay. But most important, I achieved the object of my drive: finding the freshest oysters possible, plucked directly from their chilly beds, with mere seconds between sea and slurp. An oyster that fresh is as sweet, salty, and bracing as the wind whipping across the water.

Twenty minutes up Highway 1 from Point Reyes, on the eastern stretch of the bay, the Tomales Bay Oyster Company and Hog Island Oyster Company sit at opposite ends of the town of Marshall. Twenty minutes west of Point Reyes on Sir Francis Drake Boulevard (follow the signs to the lighthouse), Johnson's Oyster Company overlooks Drakes Estero, another small bay across the peninsula from Tomales. All three farms are open to the public; the oldest such farm in the area, TBOC, has operated there since 1909.

Situated on the inside of a bend in the road a couple miles south of Marshall, TBOC's bayside plot of land is easy to miss. If you're coming from Point Reyes, scan the left-hand side for a long white picket fence. On the other side is a picnic area with tables, barbecues, and shucking knives; a dock; a few sheds; and two giant cold-water storage tanks.

Tomales Bay receives runoff from many surrounding ranches during the rainy season and is subject to closure if rainfall exceeds 0.4 inches in 24 hours. For this reason, both TBOC and its neighbor Hog Island use tanks that sterilize seawater with ultraviolet rays. The oysters sit there for a day before being shipped or sold, which filters out not only pollutants but also any silt they may have collected in the bay. "It makes for a nice, clean bite," says TBOC's Drew Alden, who took over the business about eight years ago. TBOC and Hog Island also harvest clams, a logical addition to the oyster crop, and Alden claims to eat them raw too. "It's an acquired taste – acquired by Drew," says a woman standing nearby as Alden gleefully downs one, making good on his claim.

Three miles up the road at Hog Island, Terry Sawyer takes me through the varieties of oyster produced by his farm, which has acres at the mouth of the bay, where the water is colder, in the middle, and toward the warmer inside end. "The Hog Island sweetwater is our own version of the Pacific oyster," he says. Hog Island also grows Atlantic oysters, European flats, and Kumomotos, a Japanese species. The saying goes you're not supposed to eat oysters in months that don't have the letter r in them (otherwise known in the western hemisphere as summer). The reason behind the saying is that most oysters spawn during those months, which gives them a milky texture. The Kumomotos's season runs opposite to the others', meaning they spawn in winter and stay firm during the r months.

Around the bottom of Tomales Bay, over the ridge that separates it from the ocean, a traveler in search of more oysters will find Drakes Estero. As you head toward the lighthouse, a brown National Park sign suggests a visit to Johnson's Oyster Company, established in 1929. Johnson's is notably missing the cooling tanks found on the other two farms, as third-generation owner Ben Johnson proudly notes. Johnson's is located on National Park land, and the minimal runoff in the surrounding watershed (there's one cow per 80 acres) means Johnson hasn't had to invest in cold-water storage. And after 20 years of research, the farm has found a way to move its oysters around the pristine waters to keep harvesting 365 days a year. "No matter how much it rains," he says, "I can always have at least part of the bay open."

At Hog Island, Sawyer made a point of characterizing himself and the others as farmers, not fishermen: "Our tractors are really boats." Now through April is when their crop is at its best. And with luck we'll have more sunny weekends between now and then, so buy a six-pack and a loaf of bread and head north to the freshest picnic on the half shell.

If you go

Tomales Bay Oyster Company 15479 Hwy. 1, Marshall. (415) 663-1242.

Hog Island Oyster Company 20215 Hwy. 1, Marshall. (415) 663-9218.