The National Park Service wants to run a small ferryboat to connect its 100-year-old Kenilworth Aquatic Gardens with the National Arboretum directly across the Anacostia River.

Strongly supported by both the Park Service and the Arboretum, the ferry would provide greater public access to one of the nation's major water gardens, with its extensive collection of waterlilies and marshes noted for herons, horned owls, ospreys, wild rice and muskrats.

The boat, a $15,000 Boston whaler, could be donated by the Friends of the National Arboretum, which is now helping create a 150-foot waterfall and Japanese valley garden on the Arboretum's bluffs overlooking the Anacostia River, according to new Arboretum Director Henry Marc Cathey. The proposed Arboretum dock would be near the foot of the waterfall garden.

The Park Service is proposing an extensive boardwalk and small boat dock in the marshes at the Aquatic Gardens as part of its long-range development plans for the gardens and nearby Kenilworth Park. Public hearings will be held tonight and Saturday on the Park Service plans.

Under the proposals, both the Aquatic Gardens and Kenilworth Park, created a dozen years ago on top of Washington's once notorious open-burning trash dump, would get major improvements in security and public access, but otherwise would remain largely as they are.

The cost of the ferryboat operation would be minimal since private funds would build one of the docks and buy the boat. The total cost of proposed improvements at the 500-acre Kenilworth Aquatic Gardens and Park would range from $545,000 to $2.4 million, according to Park Service estimates, depending on the extent of improvements.

The grassy open areas that now cover the old dump have become the rugby center of Washington, with six fields used for local and international team play. The park also contains tennis courts and other playing fields, picnic aras and the indoor Kenilworth-Parkside Recreation Center, operated by the District of Columbia.

Among the most expensive projects in the plan are the proposed construction of two regulation rugby fields, with a covered grandstand and changing rooms, which would cost about $375,000. A boardwalk trail and observation platforms in the marshes surrounding the garden's 11 acres of lily ponds would cost about $520,000, according to estimates in the 146-page environmental assessment that is part of the Park Service development plan for the garden and park.

Cathey called the boardwalks and ferryboat "an exciting idea that would open up" the only remaining marshes on the Anacostia River in a way that would little disturb the abundant marsh bird and plant life.

Most of the proposed Park Service improvements, however, would be in roads, parking lots, lighting and fences to increase security and visitor access. The Aquatic Gardens have had relatively few visitors in recent years--only 65,000 last year compared to well over 500,000 a year across the river at the Arboretum.

The Aquatic Gardens' somewhat seedy condition and isolated location and its entrance at the end of neighborhood streets in a public housing project apparently have contributed to the low attendance and vandalism.

One proposal is to create a new entrance for both the park and gardens on parkland abutting Kenilworth Avenue. A single entrance, with new fencing, would provide better security for the park and gardens and the Park Service greenhouses, where flowers for the White House are grown.

The Aquatic Gardens, founded in 1882 by Civil War veteran W. B. Shaw and acquired by the Park Service in 1938, have been placed on the National Register of Historic Places and cited as one of the nation's most beautiful and expansive collections of exotic waterlilies and sub-aquatic plants.

Although Shaw started the project as a hobby, he later turned it into a commercial garden that sold more than 1,000 cut flowers a day. The Park Service once had an Aquatic Garden staff of 28, but it shrank to as few as six several years ago.

Kenilworth Park was part of the government's Anacostia Park and was undeveloped until 1942, when it was leased to the District as a dump. It was converted to a sanitary landfill after a child suffered fatal burns in 1968 while playing amid the smoldering trash piles.

No buildings can be constructed on top of the old dump, since the decaying trash and garbage still emit methane gas. The Park Service plans to capture some of the gas and use it to light street lamps inside the park.

Today's public hearings will be held at 7:30 p.m. at the Nevel Thomas Elementary School auditorium, Anacostia Avenue and Grant Street NE, near the intersection of Benning Road and Kenilworth Avenue. Saturday's hearing will be held at 10:30 a.m. at the National Arboretum auditorium, 24th and R streets NE.

Written comments on the Park Service plans may be made until April 5 and should be sent to Superintendent, National Capital Parks-East, 1900 Anacostia Dr. SE, Washington, D.C., 20020.