TO most people, surfers are the quintessential icons of care-free California - tow-headed youths who thumb their sun-blocked noses at 9-to-5 jobs and spend their days in briny leisure.This stereotype is probably as timeworn as Annette Funicello. There may be as many different people who surf as there are who play tennis: doctors, lawyers, teachers, and accountants. Now add another dimension to the surfer mystique: environmental activist. In recent years, concerns about oil spills, beach access, and ocean pollution have spurred surfers to become more politically active. This week, an organization of surfers and the United States Environmental Protection Agency reached agreement with two powerful pulp companies under which the mills will stop polluting waters off northern California and pay $5.8 million in fines. EPA administrator William Reilly hailed the accord as a landmark. As much as anything, the settlement underscores the growing clout of a segment of the California populace that most people take with a grain of sand. "This is not a group of young kids who hang out at the beach all day and then cruise around at night in their 'woodies, says Adi Liberman of Heal the Bay, an environmental group in Santa Monica, Calif. Surfers, in fact, have been involved in ocean issues for a number of years. The group that successfully challenged the pulp mills, the Surfrider Foundation, was founded in 1984. Its membership has tripled in the past two years, to 15,000. It is now signing up recruits on the East Coast. That activism is not limited to US shores. "Save Our Seas," a group that includes surfers, has actively fought ocean pollution in Europe. Surfers have successfully stopped discharges off Sydney, Australia. While many discharges of sewage, trash, and toxins have been cleaned up over the past decade, the problem of ocean pollution remains large: A recent study by the Natural Resources Defense Council (NRDC) found that, for 338 days last year, one beach or another in California was closed because of health concerns. In Humboldt Bay, where the two pulp companies have been dumping 40 millions gallons of untreated waste each day, surfers complain of the water turning "black." Others up and down the coast tell of becoming ill from effluents. "There is a new generation just learning about the problem and becoming impassioned as it does," says Jake Grubb of the Surfrider Foundation. Under the EPA pact, Louisiana-Pacific Corporation and the Simpson Paper Company will construct new facilities by March of 1994 to treat discharges into the ocean near Eureka, Calif. Prior to that, Louisiana-Pacific will build an extension pipe to take wastes farther out to sea, while Simpson will try to eliminate chlorine from its bleaching process or extend its drainage system. The settlement sets a precedent for governing mill effluents. Until now, pulp and other companies have been required to see that chemical discharges meet numerical safety standards. The two mills will now have to prove they are safe to several species of ocean life - a more rigorous test. Surfrider and the EPA have been fighting the timber companies in court since 1989. The victory will make surfers a more formidable - even if slightly oblique - force.