Every summer, fed on fantasies of Huckleberry Finn, my sister, my brother and I would build a raft and float it on the creek at the foot of the meadow. And every summer, until my brother grew large enough to protest, my sister and I would set him on top of the raft. Which would, every summer, sink.

Even afloat, rafts are a cool and watery escape from summer, and -- in the hands of professionals -- a good deal safer than those of our childhood.

You can put together a rafting party as a white-water adventure or a lazy, dog-day float down the river.

River and Trail Outfitters: Guided white-water raft trips, accommodating up to 40 people on weekdays and 80 on weekends -- four to each raft plus a guide. The trip starts at Millville, W. Va., and follows the Shenandoah for five miles to Harper's Ferry, where it joins the Potomac and continues for another mile or so.

Lasting about four and a half hours, there's a stop along the way for a meal of "high energy" foods: homemade corn bran muffins with apple butter, apple cider and a dried fruit and nut mix.

Depature time is about 9 a.m. and again at 2 p.m. through October. Cost is $18 a person on weekdays, $24 on weekends. No one under 10 allowed. Advance reservations are suggested. Write: 5 Valley Rd., Knoxville, Md. 21758, or call (301) 834-9952 or -9950.

Blue Ridge Outfitters: Guided white-water raft trips on the Shenandoah, starting at Millville, W. Va., and ending four and a half hours later at Potomac Wayside, Va. They also provide a picnic -- French bread, cheeses, pepperoni and salami, fresh fruits and a beverage.

They require all boaters to wear tennis shoes, limit participants to those over 10 and 60, and suggest that everyone wear bathing suits or shorts since they're going to get wet. The charge is $17 per person on weekdays, $23.50 on Saturdays and Sundays. Reservations should be made a week in advance by writing P.O. Box 456, Harper's Ferry, W. Va. 25425 or calling (304) 725-3444.

Shenandoah River Raft Ride, Inc.: A similar trip on weekends, only by Fred Schaefer. Life jackets and a lunch provided. The trip takes about four hours and costs $20 per person. Write P. O. Box 2271, Falls Church, Va. 22042 or call (703) 241-1317.

Spring River Corp: If you want to plan your own trip, you can rent rafts from this firm, 5606 Randolph Rd., Rockville, Md. (881-596) or 2757 Summerfield Rd., Falls Church, Va., (241-2818). A four-man raft, 12 feet long, is $50 for a weekend; a six-man raft, 13 feet, costs $60. Rafts come with a foot pump and life preservers. The company suggests making reservations two weeks in advance.

Potomac River Tours: "Scenic white-water float trips" only 10 miles outside of Washington, beginning below Great Falls, Md., and continuing six miles down the river.The group floats both western-style and eastern-style rafts.

In western-style, the raft is enclosed in a rowing frame, and the guide does all the work, leaving riders free to take pictures or dream Huck Finn dreams. aEastern-style, you paddle your own canoe -- or raft.

There is a lunch stop.You bring your own food while the tour leaders provide soft drinks. Children under 12 are not allowed on the western-style rafts, which cost $26 per person. You have to be 16 or over to ride the river eastern-style, or $20 per person.

Reservations should be made 10 days in advance through Potomac River Tours, Box 30061, Bethesda, Md. 20014, 530-8733.