D.C. Mayor Marion Barry yesterday announced plans to retain and renovate the Whitehurst Freeway, an elevated highway on the Georgetown waterfront used by thousands of District, Virginia and Maryland commuters.

The mayor's announcement, made during a boat tour along the Potomac and Anacostia rivers, had been expected since last month. In deciding to preserve the long-debated freeway, Barry rejected proposals to tear it down and replace it with a ground-level street, a tunnel or a lower elevated structure.

"We think the freeway can contribute to the beauty of the Georgetown waterfront," Barry said, citing proposed improvements that include ornamental arches. But he added, "Right now it looks terrible."

The city's plan, expected to cost $60 million to $70 million, largely in federal funds, drew mixed reactions from Georgetown and other neighborhood groups and from federal officials.

Several community leaders complained that the plan will not ease a traffic bottleneck at the freeway's western end, near Key Bridge at Canal Road. A Georgetown business group called for improved access to K Street NW from Key Bridge and Canal Road. Foggy Bottom groups have pressed for a tunnel to reduce traffic noise.

National Park Service officials were pleased that the city had dropped an earlier proposal for a tunnel and a ramp at the freeway's western end. Federal officials had warned that these projects would jeopardize the C&O Canal and nearby park land.

Barry's announcement marked the end of a 2 1/2-year study aimed at shoring up the deteriorating freeway, improving its appearance and lessening traffic tie-ups. Renovation work is expected to start in late 1986 or early 1987, city officials said.

Barry also cited two other transportation projects yesterday:

A $15 million to $20 million overhaul of the Key Bridge, starting in late 1985. City officials long have planned to renovate the bridge, which is considered among the most dilapidated in the area. The project is expected to begin after federal officials complete repairs of Arlington Memorial Bridge, also scheduled for next year.

An agreement among D.C., park service and Metro officials clearing the way for construction of the Metro subway system's long-delayed Anacostia station near the Anacostia Freeway (I-295), Howard Road and Martin Luther King Jr. Avenue. The agreement, which stemmed from congressional legislation last summer, provides for about a 30-acre expansion of Anacostia Park.

Under the Whitehurst Freeway plan, the four-lane highway is to be repaired, repaved and widened to include shoulders. At the roadway's eastern end, obsolete ramps, built as links to a freeway network that was never completed, are to be torn down. No change is planned in access ramps at the western end.

City officials said they rejected a proposal by the Business and Professional Association of Georgetown to replace part of the freeway with a tunnel beneath K Street chiefly because of cost. The city estimated it would cost $155 million, a figure the Georgetown group disputes.