A Gaithersburg man, clinging to a sinking ferry cable and screaming that he could not swim, apparently drowned yesterday after his fishing boat capsized in the Potomac River northeast of Leesburg, according to Montgomery County rescue officials.

The General Jubal Early ferry was crossing the river toward Whites Ferry in far western Montgomery County when the fishing boat capsized after apparently running into the cable that guides the Jubal Early. At least two persons aboard the ferry leaped into the chilly water to help the man, 39-year-old Milton Francis Williams Jr., officials said.

But the attempt was unsuccessful, and helicopters and search teams in boats were called to the site yesterday afternoon. The search was called off at dark.

Another man who was fishing with Williams, Charles W. Walden, 60, also of Gaithersburg, grabbed hold of the 12-foot aluminum johnboat after it tipped over, and was hauled to shore by Montgomery County rescue workers. Walden was in stable condition in the intensive care unit at Shady Grove Adventist Hospital in Gaithersburg, where hospital officials would not say what he was being treated for.

The two men were neighbors. Walden, a retired railroad worker, lives at 333 Westside Dr., and Williams, who had taken the day off to go fishing, lived at 357 Westside Dr.

They rented the flat-bottomed boat at a general store on the Maryland side of the river, Whites Ferry Store, and set out with a container of coffee and their fishing gear, said Malcolm Brown, the owner of the store. The store clerk offered them lifejackets or flotation cushions and they chose the cushions, according to Steve Reichart of Vienna, who was in the store at the time. The cushions are designed to be grabbed in an emergency, according to officials.

About 1:15 p.m., the men were about 150 yards from the Virginia shore when their boat apparently ran up on the underwater steel cable, which was below the surface at that point but rises to the land on both shores.

The johnboat capsized in about eight to 10 feet of slow-moving water, officials said. Walden scrambled to get on top of the boat, and Williams seized the cable beneath the water's surface, according to rescue officials.

There were conflicting accounts of what happened next -- whether the cable sank in the water from Williams' weight, or whether it lowered into the river as the ferry continued away from the boat toward the Maryland shore, letting the line go slack.

Brown, who also owns the ferry, said the ferry stopped in midriver, and fire and rescue officials said the operator called them from a portable telephone on board.

The ferry's operator, whose name was not available last night, and at least one passenger saw Williams sinking beneath the surface, rescue officials said. As he bobbed in the river, they dove into the 50-degree water in an attempt to save him.

"They really put their lives on the line," said Brian Anderson, a Montgomery County rescue worker. "In water this cold, they were definitely in danger of hypothermia," a drop in body temperature that can be fatal.

The apparent drowning was the third in the Potomac this year.