VANCOUVER, British Columbia (AP) _ William Clancey has a dream: Take a tiny fraction of Canada's abundant supply of fresh water, sell it to Americans in the arid Western states and make a fortune.

Canada, after all, has 9 percent of the world's fresh water and only .005 percent of its population. Water is a renewable resource that falls from the sky and often runs uselessly out to sea.

The big problem is getting the stuff to market. That's where Clancey and his American partner, Gerald Shupe of KVA Resources, an engineering firm in Bellvue, Wash., think they have a great idea.

Through Multinational Water & Power Inc., they want to divert water from the North Thompson River in British Columbia through a 12-mile tunnel to the Columbia River system. They would send it down the Columbia to a point in Oregon, then by pipeline to Northern California.

All that for a capital investment of $5 billion U.S., they say.

''We have 3 percent of the fresh water in the world in British Columbia,'' Clancey said. ''It's a great asset to us. Someday it will be a billion-dollar business.''

Not surprisingly, a lot of folks disagree, starting with the federal and provincial governments, environmentalists, Indians and many ordinary Canadians.

There are two schools of thought: One, that water is a resource like any other and should be exploited; Two, the more visceral view that water is a precious gift, above commerce.

If they feel that way, Clancy asked, ''Why don't they stop shipping gas out of the country? Why don't they stop shipping timber? We are shipping hundreds of thousands of tons of coal to Japan.''

''Canadians don't seem to be hung up on exporting nonrenewable resources, but water they are,'' said Peter Pearse, a professor at the University of British Columbia who specializes in water issues.

Huge water transfer schemes have been around for a long time, many inspired by a U.S. Supreme Court decision in 1963 that restricted California's use of the Colorado River.

The most grandiose was the Grand Canal Plan, which would have cost more than $100 billion. It involved building a dike across James Bay, the southeastern thumb of Hudson Bay, and channeling water into the Great Lakes for distribution to other parts of the United States.

An ambitious plan called the North America Water & Power Alliance envisioned diverting water from northward-flowing rivers of Alaska, the Yukon and British Columbia through the Rocky Mountain trench to the southwest. That was another $100 billion baby never born.

Other than cost, the main drawbacks of such projects were the vast alterations to the environment they entailed, with unknown consequences.

Other notions include sailing tankers into British Columbia fiords, filling them with water and transporting it down the Pacific Coast. One group even suggested towing huge water-filled bladders south to California cities.

Clancey says the beauty of his plan is its simplicity and the small amount of construction required. Most of the transportation would be natural, via rivers.

Many Canadians believe 250 million thirsty Americans lust after Canada's water, but that does not appear to be the case. Virtually all water-export plans are ''Made in Canada.''

''It's an idea of the 1960s,'' Frank Quinn, an adviser on water policy at the federal Environment Ministry, said of the Clancey plan. ''If it was going to happen, it should have happened then. Right now, there is just too much resistance.''

A debilitating six-year drought has just ended in the American West, but the region's water resources remain overtaxed.

''If sound economy reigned, there would be no market for Canadian water in the United States,'' said David Getches, a water law specialist at the University of Colorado law school. ''But sound economy hasn't been the rule for making water-related decisions in the United States in the past.''

He said the Western water shortage is both fact and myth:

''It is real against a backdrop of artificially created demand, demand created by a plethora of golf courses, elaborate car washes and exotic plantings all over Southern California. If you took away the lawns, there would be enough water for three or four times the population.''

Ray Hart, California's chief of water planning, said he gets a proposal for a major scheme about once a year. But he predicted California would have enough water despite an expected growth in population from 30 million to 49 million by 2020.

Needs will be met through through conservation and reducing marginal agriculture, and ''I can't see us going out of state before doing something with our water in-state,'' he said.

''The issue is what the cost of water is going to be,'' said Duane Georgeson, assistant manager of the Los Angeles Metropolitan Water District.

Clancey's first problems are closer to home.

The North Thompson Indian Band opposes his idea, fearing its effect on salmon spawning. So does the British Columbia government, even though Clancey claims the project would pour $300 million a year into provincial coffers.

John Cashore, the British Columbia environment minister, has extended a moritorium on water exports until May 1994, ''to enable us to deal with a major process toward revamping all water legislation in the province.''

''We have to look at all the trade issues, the environmental issues and bring it together into one comprehensive policy,'' Cashore said. ''Why should we be filling swimming pools down in L.A.? I just don't like it.''

Federal officials oppose bulk water exports, but there is no specific law against it. Quinn of the federal Emvironment Ministry believes, however, that Ottawa has the necessary authority within such laws such as the International River Improvements Act.

Clancey is undeterred.

''Do you think if the United States needs water, Canada is not going to give it to them?,'' he asked. ''When water is going out to sea by the billions of gallons, I think the government is going to be fair about selling water to its neighbor.''