California's epic quest for water, made more pressing by a western drought and a cutback in its Colorado River supply, is turning toward what many see as an obvious source: the Pacific Ocean.

For the most part, desalination has long been prohibitively expensive as a source of drinking water in California. But rising demand, dwindling supply and new technology that makes it cheaper to remove the salt from seawater are changing the economics of desalination.

"It is expensive, but it's not something of the other world anymore," said Adan Ortega, a spokesman for the Metropolitan Water District of Southern California, which serves 18 million customers.

The MWD is approving plans to subsidize five desalination plants, proposals that were submitted by local water agencies. Together, the plants could supply as much as 7 percent of MWD's customers by 2007.

"Even though it only represents a small portion of the water we use, it's an additional supply," MWD Chairman Phillip Pace said.

The MWD tentatively approved the proposals in December and expects construction to begin by 2005, pending environmental reviews. The five plants are expected to cost between $70 million and $300 million each.

Elsewhere, a plant in Florida's Tampa Bay opened last week, with a second one in the works. Texas is researching desalination sites, while New Mexico wants to produce drinking water by wringing salt from its brackish underground water.

Critics say desalination remains too expensive because of the power required to run the plants, and that it damages the environment.

For every two gallons of water filtered, one gallon of drinking water is produced. The highly concentrated salt water returns to the sea. In heavy concentrations, that brine can kill small sea creatures, according to the California Coastal Commission. Scientists are still studying its effects on dolphins and other mammals.

Supporters, however, say desalination is a crucial part of California's search for new water sources.

The state's population is expected to grow by 6 million by 2010, and last month the federal government rolled back the amount of water California can draw from the Colorado River, which is shared by six other western states.

"There's only so much you can conserve," said Steven Erie, political science professor and water expert at the University of California at San Diego. "The future is recycling and desalination."

More than a dozen small plants were built along California's coast during the early 1990s when the state faced its last drought, but most were for industry. Nearly all were shut down or dismantled because of high operating costs and because water agencies found cheaper water elsewhere.

According to the Coastal Commission, only Marina, in north-central California, still uses desalination to provide a portion of its domestic drinking water.

But the process has become more efficient in recent years, reducing energy use and costs.

In November, California voters approved $50 million for desalination plant construction and research. In addition to the five new plants under consideration by the MWD, eight others have been proposed along the California coast.

Steve Myrter of West Basin Municipal Water District holds a filter used to remove salt from seawater at a demonstration plant in El Segundo, Calif.Contract employee Rahmat Gunawa takes a water sample from the output of a seawater filtration system at the demonstration plant in El Segundo.