A sailboat was aground in the Potomac below Wilson Bridge the other day, the skipper fuming.

He'd driven onto a sand bar off Belle Haven Marina, ignoring a clear channel 100 feet away, but instead of faulting his own boat handling he blamed submerged grasses growing in the sand, which in the end halted his progress.

"Damned hydrilla," he said.

Fisherman Mack Lloyd, watching from shore, chuckled and offered another view.

"It's a blessing from heaven," Lloyd said of aquatic grasses choking the marina slips. "Maybe not for boaters, but the fishing today is what it was when my father and grandfather were kids, and the ducks and geese love it."

Shallow portions of the Potomac between Alexandria and Mount Vernon have undergone a transformation in two years, going from decades of near barrenness to an explosion of greenery as more than a dozen aquatic grasses, including hydrilla, took root. Beds of grass covering hundreds of acres are visible at low tide every day.

But is the greening a blessing or a bane?

"It all depends on who you talk to," said Jim Rasin, aquatic biologist for the Interstate Commission on the Potomac River Basin.

On one side are marina operators, waterfront property owners, sailboaters and waterskiers who say thick stands of submerged aquatic vegetation (SAVs) are an eyesore and a nuisance.

On the other are fishermen, hunters and environmentalists who say the grasses herald a river revival, providing food, cover and oxygen for wildlife, controlling sediment and stabilizing the bottom.

Happily, after a period of wild exaggeration that included the silly assertion that hydrilla might eventually grow so thick you could walk from Maryland to Virginia, the arguments are saner.

"Where we stand now," said Mike Arnold, a waterfront property owner leading the fight to control SAVs, "is this: The return of submerged aquatic vegetation to the river is good in the abstract, but no SAV has a place in a marina or boating channel or around private docks or beaches, any more than a beautiful rose belongs in the middle of a tennis court."

Arnold and the group Friends of the Waterfront want to retain the right to keep channels and marinas clear by mowing or using herbicides deemed safe. "The 1 or 2 percent of the river that has been advanced for higher use by people should be kept clear," Arnold said, "either by us or by the Corps of Engineers."

What worries him, Arnold said, is a sense that environmentalists riding the crest of the Chesapeake Bay cleanup effort "consider the Potomac and the other tributaries unimportant except as habitat for fish and other wildlife," and will try to bar any vegetation control.

Arnold has to cross 100 yards of thick grass to get by boat from his dock to the main river channel. To him, the grass is a nuisance.

But to Nancy Rybicki, the mat of grass off Arnold's dock, in neighboring Broad Creek and across the river at Belle Haven and Dyke Marsh, is a sight to behold.

She has been chronicling the plight of aquatic vegetation in the Potomac for the U.S. Geological Survey since 1978, when there wasn't any, and is delighted by the resurgence.

On a river tour last week, she pointed out clear water uncharacteristic of the Potomac in the middle of the beds, and where open patches existed in the grass there were minnows or bluegills, bass and catfish.

Rybicki frequently leaned over the gunwale to grab handfuls of various species. She said Eurasian milfoil, not hydrilla, remains the most common grass near Washington. Hydrilla, whose accidental introduction into the river a few years ago sparked intense controversy, is second, and such other species as wild celery, coontail, stargrass, horned pondweed, sago pondweed and southern naiad are returning.

Rybicki said all the grasses benefit the river biologically, but conceded they also collect unsightly flotsam and create boating problems. Because none grows in water deeper than about seven feet, she said most of the river will remain clear of vegetation.

But despite all the open waters, some folks aren't adjusting well to life in the aquatic jungle.

Ridgway Smith of Alexandria said his outboard motor broke down six times after grass blocked the cooling intake. And boaters are towed into Fort Washington Marina every weekend, said co-owner Andy Sabin, because they stray from the marked channel and run afoul of grass beds.

"Hydrilla stinks," said Georgie Dalferes, treasurer of Tantallon Yacht Club, where the grasses haven't even arrived yet in pestilential amounts. "It's going to ruin our waterway."

"I don't see any benefits," said Katie Lang, owner of Alexandria's Old Town Yacht Basin.

But just outside Lang's office, employes Bob and Barry Gibson were pulling up grass with a long-handled rake, clearing a path for boats and citing benefits aplenty.

"It's brought back the life here in the Potomac," Barry Gibson said. "It's good for the fish and the ducks and it's great fertilizer. We put some on our flower bed and it just took off."

"We should put out a sign, 'Free fertilizer,' " suggested his brother Bob. "Let people come and pull it themselves.

"But then," he said, demurring, "I guess we'd be out of a job."