A 50-foot diesel-powered sailboat that was seized on the Potomac River Sunday with nearly seven tons of marijuana on board was part of a growing fleet of boats smuggling drugs into Maryland, Virginia and the Middle Atlantic states, federal Drug Enforcement Agency officials said yesterday.

Officials said they found 13,560 pounds of marijuana on board the boat, the Centaurus, Sunday and estimated the street value of the drug at about $8 million.

DEA officials said that since they began cracking down on drug smuggling in the Florida area about three years ago, more and more boats have been using the Potomac River and Chesapeake Bay as dropoff points for large loads of drugs.

Officer John Tugwell of the Maryland Marine Police said yesterday, "There are a lot of places up and down the bay to hide, especially on the weekend when all those boats look the same.

"We just don't have the money or the manpower to keep up with them, he added, "We all know they're there, though."

Yesterday morning a second boat-a 31-foot sailboat-was stopped by authorities heading away from the Washington area. Officials said they believed the boat was connected with the first boat, the Centaurus, but no marijuana was found aboard. However, on e man was detained on a concealed weapons charge.

A total of nine men were arrested Sunday, three on the Centaurus and six on the shore nearby, the Nanjemoy-Smith Point area of Charles County, about 30 miles south of Washington.

The three men on the boat were identified as Charles L. Moore, 29, of Charleston, S.C.; Robert D. Blair of Nashville, and William G. Dodds, 30, of Key West, Fla. Federal smuggling charges were lodged against them.

Yesterday afternoon U.S., Magistrate Paul M. Rosenberg set bond at $100,000 for Moore and at $75,000 each for Blair and Dodds. He noted that none of the men had Maryland ties and since among their listed occupations was transporting yachts in and out of the country it would be relatively easy for them to flee.

The three men also were charged by Maryland authorities with possession with intent to distribute marijuana on the state level. Charles County officials charged the six men arrested on the shore with possession of marijuana. They also seized trucks and a small motorboat from the nearby area.

All six were released yesterday afternoon by the Charles County sheriff's office on $500 surety bonds.

They were identified as: Sidney L. Parnell, 31, of Fort Launderdale, Fla. and his brother, Carl D. Parnell, 26, of Chapel Hill, N.C.; Kenneth L. O'Dav Jr.. 31, of Haymarket, Va.; Doc F. Carroll, 20, of the Nanjemoy area; William J. Martin, 32, of Key West, and Brian A. Content, 23, of Chapel Hill.

Sheriff's Lt. James Gartland said another man, taken off the second boat was being held but had not been charged.

Gartland said that members of his department and federal agents had been monitoring the Nanjemoy area since last December when they were told by local boatmen that the area was a dropoff point for drug shipments.

Gartland added that just after midnight Sunday two sheriff's deputies spotted a set of muddy tire tracks leading toward Smith Point and found the two trucks, the motorboat, several cans of diesel fuel, a gasoline generator, hammers, chisels, screwdrivers and pliers.

They then notified the other agencies and staked the area out until the Centaurus appeared about 3 p.m. Sunday.

When authorities boarded the boat they found all the marijuana in bales below the decks. Originally they estimated the total amount at three tons but amended that estimate after weighting the drugs yesterday.

The seizure and arrests apparently are part of a crackdown by local and federal authorities on drug smuggling in the Maryland, Virginia and Middle Atlantic areas. Last June nearly six tons of marijuana was seized on Maryland's Eastern Shore in two raids staged by federal customs and drug agents.

Last month, according to DEA officials, the Coast Guard seized about 10 tons of marijuana in Delaware Bay near Cape May, N.J.

Also contributing to this story were Washington Post special correspondent Chris Schauble and staff member Ken Feil .