When is a boat not a boat? In Washington, D.C., on the Potomac River, in a complicated legal argument that has dragged on for years.

The argument is over windsurfers, those 12-foot, 40-pound modified surfboards with colorful sails that have become highly popular on the Potomac River and the Chesapeake Bay in the last two years.

A partial end to the legal hassle came last week when D.C. police set up procedures that finally make windsurfing legal between Chain Bridge and Woodrow Wilson Bridge, the part of the Potomac River regulated by the District of Columbia.

But unresolved, city officials said, are questions about the safeness of swimming and other water contact sports in the Potomac because of the river's pollution. The D.C. City Council is expected to take up the issue when it reconvenes this week, officials said.

In 1972, the council banned swimming, waterskiing and other sports involving "purposeful contact with the water" because of the possible negative effects of the river's pollution on human health. When the windsurfing craze swept eastward from California two years ago, and windsurfers began launching from Thompson Boat Center in the District and several docks in Alexandria, D.C. police didn't know what to do. Was a windsurfer more like a sailboat, which is legal in the Potomac, or more like a surfboard, which is illegal?

Harbor police, who watched windsurfing enthusiasts regularly fall off the boards into the river, strongly believed they were akin to surfboards, said Lt. Jim Hampton of the harbor branch.

"Going in the water is part and parcel of the sport," said Betsy Broughton, manager of Thompson Boat Center, on the Potomac at Virginia Avenue NW. "We have a tricky, very gusty wind here, and even good wind surfers still end up in the drink."

Other windsurf promoters made a case to the corporation counsel that the crafts are boats, officials said.

"If you're good enough, you don't have to get wet at all when you windsurf," Warren Simmons, owner of a windsurf shop in Alexandria, said in an interview. "It's something that can be steered or propelled, and it transports people. It's not much different from a small sailboat."

In early 1982, the police department asked Judith Rogers, the city's corporation counsel, for a reading of District law. Quoting dictionary definitions, Rogers' April 1982 opinion said that although the question is "not entirely free from doubt," it "appears" a windsurfer is the "equivalent" of a boat, said Barry Fogel, a top aide to the Rev. Jerry Moore, the D.C. council member who chairs the council's committee on transportation and environmental affairs.

Hampton said police officials "appealed and appealed and appealed" Rogers' opinion. Then the next snag emerged.

If windsurfers are boats, then under D.C. law they must be registered with the police's harbor branch, and they must abide by a long list of requirements, including one that each boat announce its name and registration number in three-inch lettering on each side. That's not possible for the two-inch-wide windsurfers.

So police have allowed windsurfers to enjoy their sport on unregistered "boats" without making any arrests or filing any misdemeanor complaints for sailing an unregistered craft, Hampton said. The issue has been in a "legal no man's land" for 16 months, Broughton said.

Several months ago, Rogers' office told the police to implement her decision by starting to register the boards. The harbor patrol set the deadline for last Thursday, and has registered more than 30 of them, Hampton said. Instead of three-inch letters on the side, the windsurfers have registration numbers on the top, he said.

This legal resolution appears to leave a double standard, though, since it is now considered all right to go windsurfing on the Potomac, but waterskiing is banned. City officials said they already have heard from proponents of waterskiing, who want to practice their sport on the Potomac as long as windsurfers are doing so.

This may also be the start of a new round of controversy over the river's pollution, Fogel said. All sides agree that windsurfers are still falling into the water, and that is a violation of the 1972 ban on water contact sports. Moore is expected to hold hearings on the issue in the coming months, Fogel said, and will consider introducing legislation to ban windsurfing or, if the river water is found to be clean enough, to allow waterskiing and other sports.

City officials and windsurfing enthusiasts both say they don't know of any cases in which a windsurfer has become sick from falling into the Potomac. But Fogel said the symptoms of any illness--ranging from eye and ear infections to hepatitis--would take weeks to appear after exposure to the water.

Everyone agrees that the Potomac has been vastly cleaned up in the last 10 years, but there is disagreement among experts about whether it is clean enough for water contact sports, city officials said. Interior Department scientists reported last week that some underwater plants are reappearing in the Potomac after disappearing for 50 years, a sign that pollution problems are being reversed.

Specialists say the worst days for pollution are after heavy rains, when raw sewage mixes with rain water runoff that is dumped into the river. The pipes that empty sewage into the Potomac are just north and south of the Thompson Boat Center, a launching site for many windsurfers, Broughton said.

"You can call windsurfing boating, but it may result in a hazard to the health of those who do it," Fogel said. "There's a serious question here."

Larry Andrews, 19, of Sixth Street SE, like many windsurfing enthusiasts, says he doesn't care much about the legal issues because he loves the sport too much. "When you're going really fast, I can't think of a better thrill," said Andrews, a sophomore painter at the Corcoran School of Art who started windsurfing this summer after getting a temporary job at Thompson Boat Center, which rents the boards. "You're so close to the elements, and when the wind's howling, your whole body's almost parallel to the water. It's incredible. I was out almost every day this summer."

Andrews said he's not much concerned by river pollution, either. "I think the water's fine," he said. "On real hot days, when the water gets stagnant, you can see the algae go by on the top. That's as bad as it gets."