Kent Mountford, an environmental scientist, gazed across the choppy Potomac River last week after a leisurely mid-afternoon cruise.

"We're only now seeing the healing process," he said. "The Potomac River is really coming back."

Mountford's upbeat comment marked the end of two days of debate, speechmaking and sightseeing by more than 100 government officials, scientists and activists. They gathered here to discuss the Potomac -- the target of a $1 billion clean- up drive launched by President Johnson in 1965. Though the mood was largely optimistic, there were undercurrents about mounting costs and environmental uncertainties.

The mixed feelings were reflected in Mountford's talk as he guided the group on a short trip along the river in a tour boat. He pointed to jumping fish and a lone wind surfer as signs of improved prospects for aquatic life and river sports. He noted the clarity of waste water discharged from the Blue Plains sewage treatment plant in Southwest Washington.

Yet Mountford, an official of the District of Columbia's Department of Environmental Services, also cautioned against permitting swimming in the Potomac because of uncertainties about the level of disease-causing organisms. He cited the river's brownish color as evidence of silt and other substances washed in by land erosion and runoff upstream. He complained about continuing pollution in the Anacostia River, which joins the Potomac at Hains Point.

Most government officials and scientists say that marked improvements have occurred in the Potomac in recent years. Algae mats no longer float on the river. Raw sewage is far less evident.

Dissolved oxygen, an absolute necessity for fish and other river life, has increased. Disease-causing bacteria appear to have decreased. Discharges of phosphorus and several other major pollutants have been curtailed sharply.

Nevertheless, regional officials are currently conducting an extensive reevaluation of the Potomac cleanup effort. They are seeking to cut costs and revise their strategy as a result of new scientific data and environmental analyses. This reexamination was repeatedly evident during the conference, sponsored by the Interstate Commission on the Potomac River Basin.

Fairfax County Executive J. Hamilton Lambert called for "more realistic standards" governing discharges into the river from sewage treatment plants. The existing standards, he argued, "were not adequately reviewed at the time they were established" in the late 1960s and early 1970s. Fairfax and other local governments are pressing for more lenient regulations.

Austan S. Librach, environmental programs director for the Metropolitan Washington Council of Governments, suggested that it may be "possible to relax some treatment requirements under certain conditions and still continue to improve [Potomac] estuary water quality." The council is conducting a computerized study, which already has raised doubts about long-standing cleanup goals.

Robert S. McGarry, general manager of the Washington Suburban Sanitary Commission, which serves most of Montgomery and Prince George's counties, proposed a new goal for the Potomac cleanup. He urged that pollution be reduced enough to keep the river as clean as it was in the mid-1940s. McGarry's suggestion was later challenged by Mountford, who cited statistics indicating high bacterial counts at that time.

Officials also expressed concern about whether a relaxation in the Washington area's sewage treatment standards might damage the Chesapeake Bay, into which the Potomac empties. At the same time, they suggested that steps may be needed to curb pollution stemming from drainage and storm runoff from agricultural fields and urban areas farther upstream.

Most officials voiced optimism. "Take a good look," Mountford said, as the boat put about west of Key Bridge. "It's really a beautiful river here and something we can have affection for."