THE WATER runs clear except after a rain and, then, a mist hangs low over the James River valley, just enough to soften the focus and add mystery. Poplars and oaks line the banks and paint the water green. Civilization is screened off.

The sky, clearing from a storm, is a mountain of clouds that thins and changes shapes at the edges. Now and then, the sun burns a tunnel through the mountain and reveals a bright blue sky.

A great stone island rises out of the river, a castle inviting exploration, but there is no hurry about getting there. Lay back on your round rubber yacht and let the river crew. It will carry you gently and cool you on a summer day.

In Idaho, kids ride inner tubes down irrigation ditches. In Wisconsin, it's the Apple River for tubing. In Virginia, the James River above Scottsville has become a magnet for lazy summer floating.

As tuber Ann Etchison of Charlottesville says, "It is beautiful, very green and very peaceful. Very serene." Tubing the James is also convenient, thanks to Christie and Jeff Schmick's James River Runners at Hatton Ferry, about 30 miles south of Charlottesville.

James River Runners operates out of an 1883 store building, renting canoes and rafts as well as inner tubes. The $6 tube rental includes a bus ride to the point, where the float starts, and an orientation talk that includes an admonition to wear tennis shoes and a request to police trash. The float ends right at James River Runners property.

For most of the three miles, the river is serene, with a few little riffles. The water is about 21/2 feet deep in the summer and, except after a rain, so clear that floaters are encouraged to use goggles and face masks to look for "treasure" on the river bottom.

Sometimes they find it, according to Schmick, including wallets, tackle boxes and knives.

The slow-motion ride comes as a surprise to some customers. One was ecstatic when he got back to the store. "I never had so much fun," he announced to one and all.

"I've been on rivers all my life," he continued. "I have a power boat. But it's the first time I knew you could be on a river and do absolutely nothing. It's wonderful."

Usually, the float takes between two and four hours, depending on how much time is spent exploring the river islands and the river bottom. There have been tubers, though, who tarried so long en route that search parties were sent out to bring them in before dark.

The Schmicks got into the tubing business about four years ago when, as Jeff Schmick tells it, "I saw these two guys get out of the river here with two big black inner tubes and the idea hit me."

Business has been growing ever since, mostly by word of mouth. Normally, about 500 customers a week come to ease down the river, but on Saturday a couple of weeks ago, more than 300 showed up on Saturday alone. Reservations can be made by phone and are suggested for weekends.

To preserve the peaceful atmosphere on the river, the Schmicks stagger the trips, taking small groups upriver about 20 minutes apart.

For $3.25 you can also rent a tube to float your cooler. For a change of pace the Schmicks also have a "floating island," an oversized float that can hold four people and a cooler and rents for $25. And for those who can't stand doing nothing on the river, they also have rafts and canoes to rent.

Customers must wear tennis shoes on the river, and non- swimmers and small children are required to wear life jackets. TUBING DOWN THE RIVER

James River Runners is in Scottsville, Virginia. For information, write Route 1, Box 106, Scottsville, VA. 24590 or call 804/286-2338.

GETTING THERE -- Take the Beltway to I-66. Pick up U.S. 29 south to Charlottesville. As you enter the city, take the entrance ramp for U.S. 250 West/U.S. 29 South toward I-64. Follow I-64 east (toward Richmond) to the 24A exit, which is Route 20, and drive south 18 miles towards Scottsville. Turn right on Route 726, where there is a James River Runners sign. Then, straight to the second stop sign. Turn right. Go three miles. Turn left at another JRR sign, onto Route 625 to Hatton Ferry. RIDING THE RUBBER DOUGHNUT

Tubing is an unorganized sport, one of the last -- there are no guide books and apparently no national organization -- and it doesn't cost much.

All you need is a sound tube that fits, not so large that you fall through; an old pair of tennis shoes; a life jacket in whitewater; and a stream. Oh yeah, and a friend to remind you to put the stem side down.

The number of places to tube, though, is limited by access problems, by law or by danger. Two that are popular and accessible are the upper Gunpowder above Baltimore and the Shenandoah at Harpers Ferry.

Tubing on the upper Gunpowder offers 3.3 miles of fast, cold smooth water through Gunpowder Falls State Park, from Masemore Road to Big Falls Road.

Judging by the numbers, the most popular tubing place in this area is the whitewater Shenandoah at Harpers Ferry.

"It is fun, but you also need to be more careful," says raft guide Jamie Welsh, who also tubes.

"There is a lot of debris and rubble in the river from old dams and bridges," she says, "and you have to stay away from it. It is also a wide river and some of the water is very fast and powerful. You can get popped out of your inner tube real quick and you definitely should wear a lifejacket."

Also, from her experience, Welsh suggests:

*Don't tie tubes together. The rope can tangle around you.

*Don't try to stand up in fast water if you lose your tube.

*Wear tennis shoes and take sun block, insect repellent and drinking water.

*Steer clear of the rafts, kayaks and canoes, especially at rapids. They may not be able to avoid you, and they are bigger.

*Be aware of the water level.

The water level for the Shenandoah near Harpers Ferry is available from a Weather Service recorded report at 301/899- 7378 (listen for the Millville reading) or by asking the park rangers. To check on Gunpowder water conditions, call Pretty Boy Reservoir, 301/329-6880.

If you're going to try the Shenandoah, there's a truck with rental tubes right on U.S. 340 east of Harpers Ferry between the Potomac and Shenandoah River bridges. The rental fee ranges from $8 to $12; you can buy one for $24. Ask for a lifejacket if one is not offered.

If you're going to tube the Gunpowder, you'll need to bring your own tubes. Most people will be most comfortable in a standard passenger-car tube of 15 inches. Tubes can be bought in the Washington area at a number of tire specialty stores, including Merchant's Tire Centers and NTW; prices usually vary between $12 and $18. But right now NTW has a special sale on rafting tubes for $5.

GETTING THERE -- To get to the Gunpowder, take I-83 north from Baltimore to the Hereford Road exit. Go west on Hereford to Masemore Road, the first right, and follow it to the parking lot. To leave a car at Big Falls, the end of the tube trip, go east on Hereford to York Road and turn right. Take the first left at Monkton Road and bear left onto Big Falls Road to the river.

To get on the Shenandoah at Harpers Ferry, take the Beltway to I-270 to U.S. 340 west at Frederick; park near the park entrance in Harpers Ferry, just beyond the bridge. Then, either start under the bridge and float down to the park beach or walk up the railroad tracks about a mile to Bull Falls, which you will recognize by the congregation of people and boats, and float down to the bridge or the beach.