Alexandria and Fairfax County tax officials have begun a crackdown on residents - apparently hundreds of them - who keep boats at Potomac River marinas and do not pay personal property taxes on them.

The taxes, at the same rate as automobiles or roughly $1,000 a year for a $200,000 boat, must be paid on any boat kept or docked within the city and county during the major portion of the year. Tax officials say this means the vast majority of boats at the half dozen Potomac River marinas within their jurisdictions.

However, while some yachtsmen conscientiously pay annual property taxes on their boats others claim they don't have to pay because the boats are in District on Maryland waters (the District and Maryland have no personal property taxes) and therefore are not subject to any Virginia taxes. The Virginia border hugs its Potomac River shoreline, so most of the river belongs to Maryland [WORD ILLEGIBLE] the District and is patrolled by their marine police.

"In our opinion this is District water and the city has no jurisdiction over us," the commodore of one Alexandria yacht club said. "The property tax for cars we can understand because the city maintains roads and streets, but Alexandria has nothing whatever to do with the Potomac River. D.C. Harbor police and the U.S. Coast Guard patrol the waters and inspect our boats and most of our boats are registered in the District.?

Alexandria last fall sent tax agents onto the docks of one city marina, the Old Town Yacht Basin, to take numbers of those boats with Virginia registrations. The docks have since been padlocked, with entrance permitted only to boat owners.

"We tagged 17 boats before they locked the docks," says Alexandria Finance Director Howard J. Holton, "and found nine were Alexandria residents who had not paid. We sent them letters and then second letters in April and still have had no response, so we're going to take legal action against them shortly."

However, while city officials checked those boats with Virginia registrations, the vast majority of the 150 boats at the Old Town Yacht Basin are registered in the District and Maryland, as are most of the 750 boats in other Maryland and Fairfax marinas. Both city and county officals admit no effort has been made to determine who owns the boats and if taxes are being paid on them.

Paul Smith, director of Fairfax County's personal property and state income tax division, said this week, "We are asking our marinas for a list of all boats - something we used to do but haven't done for several years - and we're going to check those registered with the District and Maryland, something apparently we have never done."

Alexandria's Holton said his tax agents soon will be visiting other marinas in the city, particularly the Washington Sailing Marina just south of National Airport, which is operated by a concessioner on National Park Service land. The largest local marina, it has about 500 boats, most of them on trailers in the marina's parking lot, where owners cannot argue that they are in District or Maryland waters.

Assistant city attorney Burton Hanbury Jr. says that, while there has been no legal test of the issue here, " boat tied to a pier jutting out from our land are considered to be located in Alexandria for tax purposes, if they are here for the major portion of the year." This is true regardless of whose "waters" the boat actually is floating in or where it is registered, says Hanbury, and is not affected by Alexandria's longstanding legal dispute over whether the city or the federal government owns much of the waterfront.

The "waters" issue, however, is murky and also confuses boat registry. Most small boats are registerd by state, bearing state abbreviations and numbers on their bows, while larger ocean-going yachts traditionally are "documented" by the U.S. Coast Guard and bear only their names and home port.

D.C. Harbor police require all boats at the Washington Sailing Marina for more than 60 days to obtain DC registration, because the docks stand in District waters. But they do not inspect boats along Alexandria's downtown waterfront because "the D.C. boundary ends at the pier line, so we don't go in there," say harbor police officers. Boats in these marinas can be registered in the District or Virginia.

However, while D.C. police may not consider these dock to be in District waters, the federal government - and many boat owners - claim the water and much of the land along the shore was within the District's borders in 1791 and still belongs to the District or federal government.

"It's all a large problem now in the courts," says Old Town Yacht Basin proprieter Kathryn Lang. "We don't have anything to do with taxes or registration. These aren't Alexandria or Virginia waters, these aren't Alexandria docks and even the land doesn't belong to Alexandria," says Mrs. Lang.

At the Belle Haven Marina, also a concession run on National Park Service land, Newvy Laurus, head of the mariner sailing school, says "technically it's legal to register boats in Virginia or Maryland but if boat owners want to be absolutely sure they should register in Virginia." As far as paying property taxes on the boats, all marina operators and commodores say that is strictly a decision for the boat owner.

Fairfax taxman Smith says "the maps I've seen all show our marinas to be in Virginia waters, but we're looking at the maps again to be sure."

"We've discussed this at board meetings," says Richard A. Buckley, commodore of the Mount Vernon Yacht Club which has about 120 boats, all owned by Virginia residents but many registered in Maryland and the District. "It's up to the individual . . . I have a 22-foot cabin cruiser and I pay. If someone hasn't declared and nobody comes to check then he's just been lucky. I figured there would be a time they would come out checking so I'm in the clear."

Many boat owners readily admit to not paying and not declaring their boats on annual personal property tax forms. They contend the Potomac is Maryland and District Water, their boats are in it, and no Virginia tax officials have ever come to any of the marinas to say otherwise.

"I don't pay and I don't intend to pay until they take this to court, because I don't think they have a legal leg to stand on," says one Alexandria resident. In the mean time he has registered his boat in the District because he feels this bolsters his contention his boat is in District waters and also because he admitted, it makes it harder to trace.