The raft trip begins in froth and thunder just below Great Falls, where the Potomac River has a dangerous look and the sound of doom. Our whitewater guide's final instructions did nothing to dispel that ominous impression.

"If you fall in the river and get pulled into a whirlpool . . . " began Bob Marshall, who kept a devilishly pleasant smile on his face while discussing ways to avoid an unpleasant death in water. The overall effect convinced Elissa Fisher that her mild panic should be upgraded into something closer to terror.

"Do we really have to do this?" asked Fisher, sitting on the rubber bottom of a six-person raft that is floating safely beside the Virginia shore. "I think this would be a great place to spend the day."

Whitewater rafting is much like riding a roller coaster. It should take your breath away, but not permanently. On the most challenging rivers, it is sometimes hard to tell where fear ends and the fun begins.

The Potomac, after the beginning shock of water from the falls, is relatively tame as whitewater-rafting rivers go. There are three or four drops and a few whirlpools to keep the novice wide-eyed and soaking wet. But from the safety of a raft, it is as much a scenic float as a whitewater tumble.

Compared with rafting on rivers like the Cheat in West Virginia and the Youghiogheny in Pennsylvania, the weekend trips conducted by Potomac River Tours are strictly beginner runs. But the Potomac has other advantages. Just below Great Falls, the river runs through Mather Gorge, which seems too wild and rocky to be 10 miles from downtown Washington.

"Coming here means no flat tires in the middle of West Virginia," said Frank Sandrovich, a Prince George's County policeman who has been on other raft trips on wilder water. He chose the Potomac last weekend to introduce his sons, 11 and 13, to their first raft ride because of its relative tameness. Not wanting to spoil the fear-fun for his kids, however, he gave them his own last-minute encouragement--"It will be just like 'Deliverance'."

There are 26 people on this five-hour trip. Many are parents of children who attend Glenbrook Nursery School in Bethesda. "I took my nap, ate my brownies and got five gold stars so they let me come," said Arthur Bjorlykke, the father of Elissa the Fearful, whose wit stayed bone dry during the wettest parts of the trip.

"You should have seen when I took Elissa on her first merry-go-round ride," said Bjorlykke, who claimed to have ordered up our gray day to keep his bald head from burning. "There was yelling and screaming . . . but she took me along, anyway."

Potomac River Tours is the creation of Bob Marshall, a 34-year-old engineer with the Environmental Protection Agency. Marshall began taking friends on raft trips in 1976. It wasn't until 1980 that the protective National Park Service licensed him to begin the first, and only, commercial operation on the river between Great Falls and Washington.

Now, with an irregular staff of 25, PRT offers kayak and canoe classes and two weekend raft trips. Our trip was a six-mile run for $26 each. A longer trip continues past Little Falls and costs $38 per person.

"We have to comply with an unbelievable number of regulations," said Marshall during lunch beside a waterfall on the Virginia side of the river. The lunch break came after the rapids of Difficult Run, Yellow Falls and the Maryland Chute, and scenic beauty that surprises people on the trip, many of them longtime residents.

"In the fall, this river is a great migratory highway for birds," said Tom Springer, a 49-year-old guide who has seen deer, beaver, eagles, hawks and buzzards while steering rafts down the Potomac. Springer points out geological formations in the cliffs of Mather Gorge and sprinkles in a bit of river history during the trips. At one point, we passed an iron boat ring hammered into a rock wall during the Revolutionary War. Later, we rode below the remains of the Pawtomack Canal, built by order of George Washington and abandoned more than a century ago.

"Up here, watch out for the guys with theneedle guns," said Springer, teasing Elissa the Fearful, who became more comfortable with each passing rapid.

By the end of the trip, the paddlers who began with tentative strokes were confident enough to start water fights and to ram other rafts. People were getting dunked and enjoying it.

"This is a chance for parents who left the kids at home to act like kids," said Jim Weiss, a Washington attorney. "It's sort of like bumper cars on water."