AMY TAXIN  / Associated Press

For more than two decades, California’s Orange County has debated whether to build a seaside plant to convert the Pacific Ocean’s salt water into drinking water in hopes of buffering against droughts like the one now gripping the nation’s most populous state.

On Thursday, the $1.4 billion proposal by Poseidon Water was given a critical review by the California Coastal Commission, which is tasked with protecting California’s scenic shores. A vote was expected later in the day.

Poseidon and its supporters, including Gov. Gavin Newsom, contend the Huntington Beach plant will produce 50 million gallons of water a day that are crucial to help weather cutbacks on state and federal water supplies following years of drought. Newsom, a Democrat, recently told the Bay Area News Group editorial board that a denial would be a “big setback” and “we need more tools in the damn tool kit” to address drought.

“The need for this project is critical and without question,” DJ Moore, an attorney for Poseidon, told the Coastal Commission. “California is at an inflection point on water.”

But environmental groups and the commission’s staff, which reviewed the plan, oppose it. They argue it will damage marine life by killing tiny organisms that form the base of the ocean’s food web. They also say it’s vulnerable to flooding and other hazards. And some in the water industry say the cost of desalinated water is too high and isn’t needed in an area with access to cheaper sources of water.

“The project would kill marine life in about 275 million gallons of seawater per day,” Tom Luster, the commission’s desalination expert, told the panel.

The long-awaited meeting before the 12-member commission is seen as the last big hurdle to the plant’s construction. The panel heard hours of comments from scores of people packed into a hotel meeting room in the Orange County city of Costa Mesa on Thursday in addition to those tuning in online.

At the meeting, supporters wore orange and yellow construction vests and toted signs saying “support desal!” Opponents carried signs reading “No Poseidon” and “Do not $ell our coast” and included a woman who wore a plankton costume and held a sign reading “I am a plankton — please do not kill me!”

California has spent most of the last 15 years in drought conditions. Its normal wet season that runs from late fall to the end of winter was especially dry this year and as a result 95% of the state is classified as in severe drought.

Newsom last summer urged residents to cut consumption by 15%, but since then water usage has dropped by only about 3%. Some areas have begun instituting generally mild restrictions such as limiting how many days lawns can be watered. More stringent restrictions are likely later in the year.

Much of California’s water comes from melting snow and with a far below normal snowpack, state officials have told water agencies they will receive only 5% of what they’ve requested from state water supplies beyond what’s needed for critical activities like drinking and bathing.

Desalination takes ocean water and removes salt and other elements to make it drinkable. Those elements are discharged back into the sea, while the water can be channeled directly to consumers or used to replenish a groundwater basin. The country’s largest seawater desalination plant is already operating in nearby San Diego County, and there are also coastal plants in Florida.

The idea of desalination has been debated for decades in Huntington Beach, a coastal community southeast of Los Angeles known as “Surf City USA” that relies on its sands and waves for tourism. Discussion of the project has also recently focused on the impact of climate change on regional water supplies and on sea level rise in the low-lying coastal area where the plant would be built.

More than two decades ago, Poseidon proposed building two desalination plants — the one in San Diego County, and one in Huntington Beach. The San Diego County plant was approved and built, and desalinated water now accounts for 10% of San Diego County Water District ‘s water supplies.

But the Huntington Beach project has faced numerous delays. In 2013, the Coastal Commission voiced concerns that the proposed use of intake structures to quickly draw in large volumes of water from the ocean would damage marine life. Poseidon, which is owned by Brookfield Infrastructure Partners, conducted additional studies and resubmitted the plan with a proposal to mitigate marine damage through restoration of nearby wetlands.

Last month, staff members for the panel issued a 200-page report opposing the project, arguing it fails to adhere to marine life protection policies and policies aimed at minimizing hazards from tsunamis and rising sea levels. Environmental groups have raised numerous objections and oppose the use of intake structures that pull water for desalination from above the ocean floor instead of beneath it.

“This proposal is so fundamentally flawed that no amount of mitigation credit is going to fix it,” Sean Bothwell, executive director of California Coastkeeper Alliance, told the panel. “It’s a disaster.”

Some on Thursday also debated the extent of the local demand for the desalinated water. Orange County has an ample groundwater basin and recycles wastewater, making the region less dependent on imported water than San Diego. The Orange County Water District, which has said it intends to buy Poseidon’s water, manages the basin that helps meet about 75% of the water demand in the northern and central parts of the county.

Poseidon contends the region would still benefit by locking in a drought-proof source of water and so would inland communities and states that could gain increased access to imported water supplies once the county can tap into desalinated water. Steve Sheldon, the Orange County Water District’s president, said desalinated water is more expensive now, but he expects the cost of imported water to also rise over time.

Critics argue the area would be better served economically and environmentally by focusing more on recycling, noting an expansion of the county’s renowned wastewater recycling program is already under way. Paul Cook, general manager of the Irvine Ranch Water District — a local water retailer — said he doesn’t want to buy pricey and unnecessary desalinated water for his customers, driving up household water bills.

But Kenny Williams, president of the Orange County Labor Federation, said while the plant’s impact on marine life and other hazards can be addressed, the drought is a threat to Southern Californians’ way of life.

“We can afford a few more dollars on our water bill. We can’t afford the taps going dry,” he told the commission.