WITH CAPTAIN BRIAN LATHAM at the wheel of the van, we swing off the Toulouse-Carcassonne highway, go bouncing along a rutted dirt road to enter into the pastel mood of an Impressionist painting. Sunlight filters through the plane trees to create a dappled effect on the green ribbon of water.

"There they are," our leader says with obvious pride as we roll to a stop. "Those are what you'll be calling home for the next sit nights."

Two so-called hotel barges are tied up back-to-back in the Canal du Midi, at this point a half dozen or so miles east of Toulouse. These are the "Water Wanderers."

Twelve of us (six American couples) are about to embark on a journey along the canal as far as Trebes, a village just beyond Carcassonne, a distance of approximately 60 miles. Our stop-and-go passage, passing through 52 locks in the process, is scheduled to take five full days.

Our program started in the lobby of the Hotel Le Concorde near the Toulouse railroad station at 5:30 on a Wednesday evening. My wife and I had come from Paris on the Air Inter flight. Others arrived by train (a 7-hour ride from Paris). One couple came up from Barcelona.

Now, at a little after 6, Brian advises us to leave our luggage and come aboard for a welcome drink. We gather in the lounge of Water Wanderer 1 to sip a local sparking wine and make the polite, guarded and slightly anxious chit-chat of strangers about to take off on a journey into the unknown.

Passengers range in age from about 50 to 70-plus, and appear to be seasoned travelers. Conversations are sprinkled with exotic destinations. Someone drops the name Lars-Eric Lindblad. Someone else recounts a spring spent in a villa in the Algarve. We eye each other warily, wondering: What will it be like to spend most of a week together within these narrow confines? Anticipation is mixed with apprehension.

The seven members of the all-English crew murmur reassuringly, spin witty tales of past trips, pour more wine. Then Lily, the captain's wife, adroitly maneuvers one couple at a time on a tour of inspection, gives a demonstration of how the pump toilets work, and deposits us in our quarters. With sink, adequate storage space and reasonably comfortable double-decker bunks, the arrangement is not as spartan as expected.

Dinner is served at 8. The following morning, engines throbbing, we take off. It is not long before one falls under the route's spell.

Travelers considering going on their first barge trip might well share similar expectations (and doubts). The concept seems almost too good to be true. And - of course - the handout literature paints the typical cruise in idyllic terms.

A dozen or so passengers are pictured settled on a converted barge with an interior neatly fitted out in a reasonable facsimile of a deluxe cruise ship, in miniature. Days will be spent sailing up some lazy river or on a canal meandering across France, or Holland. At night, the vessel ties up alongside a picturesque or historic town, with the opportunity for passengers to go ashore. Some evening meals are served aboard, where a gourmet chef is likely to be in attendance; other dinners are arranged for in local restaurants, so one can partake of regional specialties and local wines.

For once, reality lives up to most of the promises. Oh, there can be flaws in the operation. And being confined for so many days within fairly cramped quarters may not be everyone's idea of the perfect foreign vacation. But when conditions are right - and passengers and crew get along harmoniously - this can make for a memorable experience.

The water-borne European holiday is of fairly recent origin, and is gaining in popularity, mainly with Americans and Canadians. Six-day river and canal cruises are offered in France and Holland. Others are set up in England on the Thames, and on the rivers Severn and Avon through the Shakespeare country.

The oldest and largest operator of such cruises in Continental Waterways, located in London, with five barge itineraries on the rivers and canals of France, offered between mid-April and the end of October.

Floating Through Europe Inc., of New York, has an expanding fleet of hotel barges (mainly in Great Britain and Holland), acts as agent for others in France and serves as the North American clearing house for fleets of self-drive boats operating in France, Holland, England, Ireland and Scotland.

Hotel barge programs can vary widely as to price and level of luxury. Typical of the gamut run is indicated by the Continental Waterways offerings.

At the top of the scale is the Mark Twain, a 19th-century Dutch vessel fitted out with brass, teak, and antique furniture. The Mark Twain accommodates seven passengers in cabins with private facilities. Cruises ply the canals and rivers of eastern France, gnerally in Alsace.Price for two for the six-night trip can run as high as $1,580, plus drinks and tips, and does not include transportation to or from the cruise's departure point.

A somewhat different mood is set aboard the much large La Guepe Buissonniere, where 28 passengers are put up in 14 double cabins (without private facilities). La Guepe turns on a social scene inits spacious bar and saloon area, with dances, fancy-dress, balls and other trappings from the cruise ship lexicon. La Guepe cruises between Fontainebleau and Auxerre on the River Yonne south of Paris. Cost for two sharing a cabin for the journey (five days, six nights) comes to between $850 and $950, depending on season, with passengers picked up in Paris and returned there. This cruise is also marketed by American Express as one of its European tours.

Itineraries of the barge Palinurus take in the canals of Burgundy, passing through the vineyard country of this region rich in gastronomy and monuments. Palinurus accommodates 17 passengers in single and double cabins and costs from $780 to $940 for two sharing a cabin, including pickup and delivery to an from Paris.

Probably the most romantic cruise is that of the Water Wandereres, our two small barges traveling in tandem along the Canal du Midi. Water Wanderer 1 can put up 10 passengers; Water Wanderer 2 offers two double cabins. Main meals are taken in the dining area of the larger barge. On this trip we sail with two empty berths, an indication that not all cruises are always sold out, although generally it is advisable to book months in advance.

The round trip between Toulouse and the Mediterranean is broken up into four segments. Each leg is the standard six-night package, with price for two sharing a cabin from $830 to $920.

Almost everything about the Canal du Midi trip has a delightful old-fashioned quality to it. Graceful egg-shaped locks are still opened and closed by hand, utilizing a ratchet-wheel mechanism based on a design by Leonardo da Vinci. The canal, built during the reign of Louis XIV between 1666 to 1681 to serve as a connecting link between the Atlantic and the Mediterranean, has been called the greatest engineering feat since Roman times. Running for 240 kilometers (about 150 miles) between Toulouse and the sea, it passes through 101 locks. Our sector across Languedoc will include climbing sets of "stair" locks to a height of 600 feet.

In the old days, barges were pulled by horses. Tow paths still run alongside the canal, shaded by the high plane trees planted to protect the horses from the searing sun.

We chug along very slowly. As we approach the first lock, there is a flurry of activity. Crew members scramble to their stations. Brian maneuvers the barge into the narrow opening. Gary guides the other vessel in alongside. There is a quick passing back and forth of lines, with the young women who work in the galley now serving as deck hands. One of the crew also duplicates the barges' passage in the van, leap-frogging ahead ashore to work along with the lock-keeper in opening and closing the sluice gates.

Each lock-keeper's home bears a plaque indicating the name of the lock, such as "Ecluse de la Criminelle," along with the distance in meters to the nearest locks in either direction. Invariably each home is set on a small farm, with chickens, geese, rabbits, and always a friendly dog. It si from the lock-keepers that our young cook, Veronica Harrison, will do some of her shopping, buying the freshest of eggs and salad makings straight from the garden.

By the time we reach our third lock, passengers get off to follow on the tow path, either on foot or by bicycle. With distances between locks frequently no more than a half mile, and with 15 minutes required to pass through a lock, we have no problem keeping up with the barges. (There are a dozen bikes aboard for the passengers' use.) By noon we have tied up for lunch. Meanwhile, Veronica has prepared some tasty bread. It is 2:30 before we continue our journey.

In the afternoon, passengers set out on more ambitious forays. At times Lily pilots one of the barges. And by now passengers have also gotten into the act, serving as deck hands, helping with the opening and closing of locks, even taking turns at the wheel.

By 4:30 we have completed our day's run, a distance that could be covered by car in perhaps 15 minutes, which is what barging is all about. Now we can go into a neighboring village either on foot or by bike. Later comes a leisurely cocktail hour, followed by dinner with lengthy conversations and bridge games until bedtime.

Barges tie up overnight near Avignonet, Castelnaudary and Gram. Usually my wife and I walk into town, take in whatever sights there may be, stop at a popular cafe for a beer or coffee. Unfortunately, getting close to the "natives" is not exactly part of the French provincial scene. Even with some command of the language a visitor is not likely to do more than pass the time of day with a cafe waiter. The regulars keep lively games of cards going, engage in animated conversations, concentrate on their pinball machines. We remain on the outside looking in.

From tis standpoint, barge trips in the British Isles are quite different. There's always a cheery pub where visitors can rub elbows with local resident, get involved in the talk and goings on. On French trips the English-speaking passengers and crew tend to stay pretty much insulated from the surroundings.

Barge trips are obviously the choice of older passengers, and it's my observation that many who take these cruises do so for the same reason many others book cruises on luxury liners: the need for security. The vessels offer a safe enviornment and make foreign travel less intimidating.

On two evenings we dine ashore, once at a country hotel near Villefranche-de-Lauragais, the other at what can only be called a tourist trap in Carcassonne. One lunch is served picnic-style in a setting worthy of Renoir. An excursion by van takes us to the village of Durfort, where handwrought copper wear has been made for centuries. From our mooring in downtown Carcassonne we walk up to La Cite', the medieval fortified city that ranks among Europe's top tourist attractions.

Still, the pay-off is the ride itself.

At its best, to go by barge on the Canal du Midi is to experience a quiet thrill unmatched by most other types of travel. The rhythm of our days - hours drifting by - is ideally suited to relaxation. On this narrow slot of water we sail straight through the landscape, on occasion even below it while passing under ancient stone bridges.

A camaraderie builds among passengers, as we gradually get to know each other - the architect from San Francisco, the neurosurgeon from Portland who keeps an eight-acre vineyard as his hobby, Bud and Marge retired on Amelia Island, Ruth who lived in Rome during La Dolce Vita days and is now wife No. 2 of Dr. So-and-so. . . .

On the other hand, two days of rain can change the tenor of the trip, emphasizing the static nature of the routine; then even the marathon bridge games and interminable conversations do not help us shake off our sense of confinement.

I have other reservations. I find the breakfasts skimpy, am disappointed in the cheap vin ordinaire served with meals (rather than the regional Languedoc "country" types promised in the brochures). Our two meals ashore turn out to be a kind of rip-off but most meals aboard are extraordinary, with Veronica making a great effort to come up with true French cuisine. And to have the same sheets and towels for the entire journey seems inexcusable.

But I am alone in my criticism since other passengers appear to find little to complain about. On Tuesday morning after breakfast, as the captain prepares to transfer us to the Carcassonne railway station, most passengers seem reluctant to leave. "I'd do it all over again," one announces. "And recommend the trip to my friends."

As an alternative to the barge-cruise on the Canal du Midi, a do-it-yourself version is offered via self-drive cruisers outfitted with bunks to make for a houseboat-type holiday. These easy-to-maneuver diesel-powered craft range from 27 to 45 feet, and can put up between 2 and 10 persons at a price of from $295 to $1,050 for six days. Cost of fuel is extra. There's also an additional charge for taking along bicycles. Food is extra, too, to be bought along the way and prepared aboard, or taken in restaurants.

Blue Line Ltd. of London maintains a fleet of 60 cruisers based at Castel-naudary on the Canal du Midi, 35 miles east of Toulouse, available from mid-March to end of October. Employes give instructions in boat operation (fairly simple, since speed is only 4-5 mph), and explain procedure for getting through locks (passengers are expected to help lock-keepers open and close gates). Minimal knowledge of French is helpful since passengers must also buy their own food ashore.

A similar self-drive cruiser program is set up for Burgundy by the British owned Loire Line, using fiberglass boats putting up two to eight people at $360 to $850 for a six-day week. Canals of Brittany can be explored on boats of the English firm Caravans Sur la Men at $400 to $900. Vijn Yacht Charter of Holland maintains a fleet of 18 diesel-powered steel boats for cruising the Netherlands' waterways. All French and Dutch self-drive boats (along with others in England, Ireland and Scotland) can be booked through Floating Through Europe Inc., 501 Madison Avenue, New York, N.Y. 10022.

Floating Through Europe hotel barge programs are listed in their comprehensive brochure, free for the writing. Barge vacations aboard the Water Wanderers, Palinurus, La Guepe, Mark Twain and others are detailed in the brochure available from Continental Waterways LTD., 22 Hans Place, London W.1, England, which will also arrange for bookings.