WE WERE to cruise the Nile on the Isis--a small floating hotel, freshly painted in blue and white. Crossing the gangplank with my traveling companion one afternoon at the Luxor mooring, I had very little idea of what to expect. Cruising had not previously been on my low-budget travel agenda.

Nile cruises come in all shapes and sizes. Some last for 10 days and involve sailing upriver from Cairo to Aswan, a distance of 534 miles. The more economic and popular variety start, as ours did, halfway up at Luxor (the site of ancient Thebes and the magnificent ruins of the temple of Karnak), and sail the comparatively short distance to Aswan in five days. (These cruises also work in reverse if one wants to travel the same route in the opposite direction.)

As the passengers milled about receiving cabin assignments, I began to feel a little apprehensive. Would this cruise be a civilized, perhaps intimidating interplay of manners and types--a kind of latter-day "Murder on the Nile" without, I hoped, the murder? Or would it resemble a floating Club Med with unending supplies of wine and brown bodies--or perhaps even a giant tour bus without wheels? But my cherished vision--of being borne on the waters of the Nile past the wonders of the ancient world--sustained me, and I handed in my passport and travel voucher.

Our cabin on the Isis turned out to be small but nicely organized, with an adjoining bathroom. It was at water level, and the view from the porthole across the Nile to the mountains sheltering the Valley of the Kings was truly the stuff of dreams. We were soon installed and ready--to cruise.

The first surprise for a neophyte Nile cruiser is that most of the time one doesn't. For the first three days of the cruise the boat, operated by Hilton International, stayed in Luxor, making short day trips to sites downriver. Not that one had any time to be bored with the scenery, but Nile cruising is not (unless you wish it so) long days of drifting down the river sipping chilled Omar Khayyam rose' wine on the deck. There is too much to see and the heat to beat.

Days begin--it's not as bad as it sounds--with a 6 o'clock bell, followed by a buffet breakfast. The lemon crepes, fresh fruit and good coffee took the edge off such early rising. (Food on the Isis was plentiful and tasty, including some of the only Egyptian food we managed to find during our whole stay. Most hotels ashore serve an edible but utterly bland variant of French cuisine--lots of bread and much zucchini.) The day's expedition would leave by 7 to take advantage of the cooler hours and to beat out the other tourists. Heat can be a problem in the summer months in Egypt, but from November to March the dry heat of the afternoon is offset by delightfully cool evenings and mornings.

Sightseeing is serious business on the upper Nile, but many visitors are not at all conversant with ancient Egyptian history, with its innumerable dynasties and forgettable pharaohs. So a sympathetic and well-informed guide adds enormously to a trip.

That first afternoon afloat, some of my precruise anxieties were laid to rest. The social structure of our boat became clear. We were briskly and efficiently divided into three groups, each with a name, a guide and a seating at dinner. There were "The Sovereigns" (a group of British senior citizens organized by the Sovereign Agency), "The Ladies of Jidda" (an expatriate wives club from Saudi Arabia) and, I am proud to report, "The Individuals." It brings a glow to an intrepid traveler's heart to be approached by an admiring Sovereign and to be asked "Are you an Individual?" We were indeed, and our guide, Ashraf Zhoni, proved to be a treasure.

Helwan University in Cairo offers a degree course for guides, leading to government licensing. Only about 35 a year graduate, and their rigorous training includes courses in Egyptology and the care and feeding of tourists. This last was evident in the skillful pacing of the sightseeing and the different levels of information available to visitors with enormously varying stamina and taste.

The sights and sites of Egypt are physically enormous and imaginatively overwhelming. The luxury of Nile travel is that the boat combines transportation, accommodation and restaurant facilities. With all this taken care of and no worries about luggage or logistics, energies are preserved to absorb a much higher concentration of sightseeing than most people--certainly me--take in. With only one chance to meet the pharaohs, I didn't want to have to worry about the next camel to Aswan.

The temple at Karnak, minutes away from our moored boat that first afternoon, is the largest religious complex in the world, with an area that would encompass Notre Dame, St. Peter's and Milan Cathedral. The temple dates from around 1500 B.C. (or ''before the Christian era'' as our guide punctiliously referred to it). Its centerpiece is the hypostyle hall with its 134 pillars. The 12 largest are 79 feet high and 12 feet in diameter; a hundred men can stand on the top of one column, or so claimed the Greek historian Herodotus. Beyond the hypostyle hall, a massive obelisk set up by the unpronounceable Hatshepsut, the only woman pharaoh, divides the sky. ("Remember," said Ashraf as he explained how to say her name, "A hat and a cheap suit." I did.) Another was carted off in the 19th century, like so many other Egyptian treasures, to stand in Paris' Place de la Concorde. The immense size of the obelisk carved from a single piece of stone led to a chorus of questions. Ashraf responded with unflagging good humor, sometimes turning the tables on his flock with mock quizzes. The granite, he told us, was quarried upriver at Aswan, floated down the Nile on rafts, and moved along specially dug ditches to the building site (which, of course, is why so many temples were built close to the river).

After visiting the temples of Karnak and Luxor, we sailed downriver the next day to visit the temples of Abydos and Dendera (90 and 40 miles respectively north of Luxor) in the early morning, steaming back to our Luxor mooring again through the long hot afternoon.

The prospect the following morning of a visit to the Valleys of the King, Queen and Nobles got even the tardy promptly on deck. The incredible arid landscape hid deep tombs with wall paintings as bright as if the painters had just taken a lunch break. The renowned tomb of Tutankhamun is really the least significant tomb we visited--two smallish rooms--whereas the maganificent tomb of Seti I has more than a dozen vaulted chambers stretching 350 feet into the rock. (Of course, the glories of King Tut's tomb are safely lodged in the Cairo Museum, although to my surprise, his mummy still lies, as it has for 3,300 years, in the burial chamber.)

In the dark temples and deep, deep tombs it suddenly became obvious why we set out each day with flashlights. The electrical wiring was unpredictable to say the least, and not the tiniest ray of sunlight could penetrate that far into the earth. When the electric lights failed (and they did frequently), robed attendants would materialize like magic with candles for sale. After this had happened several times, I began to suspect a correlation between the arrival of the party in the deepest part of the tomb and the failure of the lights. But the wall paintings in the Temple of Dendera are magnificent, and without a flashlight we would have missed many of them.

Back on board the Isis, all of us were bubbling over with the wonders we had seen. Interest in things Egyptian and shared experiences on the sightseeing trips bound the passengers pleasantly together. Meals were served at large communal tables and provided an easy exchange.

A hallowed social tradition, we soon discovered, was a masquerade party with an Egyptian theme always held on the last night of the cruise. A certain team spirit soon arose among The Individuals, who determined to outdo the Sovereigns and the Ladies of Jidda. A lot of ingenuity was expended on the makeshift costumes and a lot of fun resulted, though it must be admitted that the Ladies of Jidda carried off most of the prizes. My own special favorites included the Palm Tree (a most uncomfortable-looking costume), the Liberated Harem (featuring cigar-smoking, beer-drinking veiled maidens), and a stately Cheops, builder of the Great Pyramid at Giza (complete with ceremonial beard fashioned from a bent toothpaste tube).

Many passengers bought varieties of Arab dress for the costume party when we docked at the village of Esna to visit the temple there. It was begun in 170 B.C., and by now we were calling that "modern." Especially popular were the long cotton robes called galabiyah--particularly among the shy men.

Shopping in Egypt--a pleasant change of pace from sightseeing--is a mixture of fun and harassment. The country is poor; the people are legion. Therefore every affluent Western tourist--and that is every Western tourist--is besieged by hordes of street peddlers selling reproductions of antiques, wall hangings, alabaster and painted papyrus. If bargaining is an embarrassment or makes you uncomfortable, the easy out is to shop at the boat's small store or the gift shops in the big hotels along the bank in Luxor--there's an especially pleasant one in the Etap Luxor. Their merchandise is fairly standard, but often the best available, and the prices are marked.

And for those who like to prowl looking for the good deal, Egypt is paradise. Prices usually start at about twice what one should pay, but to make one offer and quit, however good the offer, is not playing by the age-old rules of the game. We bargained for painted papyrus scrolls from the stalls around Luxor with some success. The bargaining should go back and forth with good-natured arguing so that both sides can feel they struck a good deal. Most tourists overpay anyway, so one must be somewhat philosophical. A pocketful of 25-piaster notes (30 cents) is useful when one is away from the group, for baksheesh (tipping) is universal and expected for lighting the way, pointing out a wall painting, fetching a carriage.

A little bleary-eyed from the festivities of the costume party, we concluded our voyage at Aswan on the morning of the fifth day. It turned out to be a beautiful river town surrounded by high hills, one of them the site of the Aga Kahn's rose-colored mausoleum. A farewell sail in a felucca showed us the Nile from another angle. These rotund, single-sailed craft are everywhere on the wide river, both picturesque and an essential method of transport since the Nile is too wide in most places for bridges.

Then a farewell to the invaluable Ashraf (a tip of two or three Egyptian pounds per day of the voyage is customary for guides), and the Individuals, game to the last, set off on a free-lance trip to the Aswan High Dam. By then I had forgotten my initial apprehensions. I had swallowed the cruise at a gulp. The five days had been an inimitable combination of culture and company. The individuals had become old friends, whose way I had lighted with my flashlight, who had lent us tape to fashion our costumes and helped us unmuddle our dynasties. The High Dam expedition was fun, but all agreed that monuments to the technical achievement of modern man don't light a candle to the building genius of Rameses II or Hatshepsut.