Maryland and federal law enforcement officers boarded a 50-foot sailboat in the Potomac River near Indian Head, Md., yesterday, arrested three persons and seized three tons of marijuana valued at up to $1.8 million, authorities said.

In addition, six or seven other persons were arrested by Charles County authorities at what was believed an intended landing site for the sailboat, according to an official of the U.S. Drug Enforcement Administration.

Two trucks and two other vehicles as well as floodlights and a generator were found at the suspected landing site, said Morris Davis, group supervisor at DEA's Washington District Office.

The diesel-powered sailboat, identified as the Centaurus, was boarded about 3 p.m. at a spot about 30 miles south of Washington by U.S. Customs Service agents and officers of the Maryland Marine Police and the Charles County Sheriff's Department.

Authorities declined to divulge what led them to board the vessel. They said the boat had Maryland registry, but they said they did not know where it was coming from.

The marijuana was founded in bales, below decks, officials said.

While a Maryland State Police helicopter hovered overhead, the officials said, all three persons aboard the sailboat were taken into custody without incident.

Full identifications for the men were not immediately available last night, according to authorities. None was believed to be from the Washington area, they said.

Those arrested on the boat were expected to be arraigned today before a U.S. magistrate. As of late last night no formal charges had been placed against them.

No details were available about those persons reportedly arrested on shore, and the Charles County sheriff's office said it would have no statement about any aspect of the incident until 10 a.m. today.

The sailboat reportedly was moored last night at the Naval Ordnance Station at Indian Head, under guard by both federal and county authorities.

The incident, described last night by regional customs commissioner John A. Hurley as having "all the earmarks of a major seizure," involved what appeared to be one of the largest amounts of marijuana ever confiscated in the Washington area.

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 Hurley, the Coast Guard seized about 10 tons in Delaware Bay near Cape May, N.J.

Just before last Christmas, he added, Virginia State police, working with customs agents made a "multiton seizure" in the Tidewater area.

Hurley called yesterday's seizure "an extremely effective cooperative effort" among state, county and federal authorities. A customs service spokesman said Virginia state police were also involved in the operation, apparently helping keep the vessel under surveillance.

The boat, he said, was believed to have reached the Charles County area yesterday and was "going up" the river when boarded.

In explaining the paucity of details about the boat's place of origin, the DEA's Davis said "nobody in the crew is talking."