Hydrilla, the tenacious water weed that has gained notoriety because of its aggressive growth in the Potomac River, appears to be as troublesome dead as alive.

True, the recent cold weather has begun to kill the weed, as predicted. But now mats of the dead weed are troubling boaters.

On a recent Sunday morning, 47 people who clambered aboard a Washington cruiseboat for brunch and an Indian summer river tour, had to settle for a dockside brunch. Massive mats of dead hydrilla, which had been carried upriver by the tide and southerly winds, had clogged the intake ports on the Spirit of '76 so severely that she never left the dock.

"There were a lot of very angry people," says Jim Rooney, general manager for Washington Boat Lines Inc., the company that owns the Spirit of '76 and three other charter boats that operate from Pier 4 on the Washington waterfront. "We gave them a full buffet for $5 instead of $15.95. But they had come down for a boat ride."

"It's getting terrible. We used to watch for logs; now we're more concerned with the hydrilla," says Capt. Bob Pennington, who charters an 85-foot yacht out of the marina adjacent to the Washington Boat Lines.

Both Rooney and Pennington say they fear that the hydrilla, which scientists say can grow up to an inch a day in warm weather, will establish itself in the river. In Florida, where it was first discovered in the 1960s, hydrilla has taken over entire lakes, making recreational use impossible.

"If it reroots and and grows at the rate it's . . . capable of, the entire waterfront could be in trouble next year," says Rooney. "Any kind of boat that draws in water to propel it or to cool its engine could get stuck."

Richard Hammerschlag of the National Park Service says that hydrilla's tendency to mat is another of the drawbacks that have qualified the plant for entry on the federal noxious weed list. "You don't see other subaquatic weeds floating around in mats. Hydrilla is a very thick material and other things tend to become entwined in it, even when it's alive."

It is "likely," Hammerschlag says, that hydrilla will be spread to areas where large mats of its dying vegetation travel.

Rooney says the potential hazard hydrilla poses for commercial boats along the Washington waterfront is compounded by the absence of tugboats that could rescue entangled ships. "The John Glenn is the only possible tug," says Rooney, pointing to a big red District fire boat moored to an adjacent pier. "And it runs on sea suction."

Bob Friedel, fleet captain for the Washington Boat Lines, says that a cruise from Georgetown to Alexandria that normally takes 45 minutes took 2 1/2 hours recently because he had to stop every 10 minutes to clear hydrilla that had clogged the boat's intake valves. "I've got a bucket of the stuff. When the tide shifts it breaks it loose in huge mats."

"Most of the year the plant's not floating around," argues Hammerschlag. He says that public anger over hydrilla unfairly eclipsed the good news of the Potomac's rebirth.

"It may be that hydrilla will come to dominate the scene," he said, but in the meantime, "people should be encouraged to go out and enjoy the Potomac."