There are two surprises about this cruise . One is where it went, and the other is the cruise line.

If you drew up a list of the countries you are least likely to visit on holiday, they would probably include most of those on this journey down the west coast of Africa. Benin, anybody? Togo? And if you had to think of a tour company to run such an intrepid voyage, Saga might not immediately come to mind. How wrong you would be – as I was – on both counts.

In Benin, for example, we were taken by canoe to Ganvié, where a population of 30,000 lives in a town built on stilts in the shallow waters of Lake Nokoué. Ganvié dates from the 17th century, when the Tofinou people fled to the lake to escape slavers. Transport is entirely by canoe. Amid the houses, schools, shops, hotels, hospital, churches and mosques, all standing up to their knees in the water, artificial islands have been made so children can learn to walk.

And in Togo we came under the spell of voodoo. In a field in the village of Hlandé, men performed a voodoo dance to an unremitting salvo of drumming. A swaying choir of women and children clapped and chanted as giant effigies atop tent-sized skirts of coloured grass were moved about the field. Each took six men to lift. At the dance's climax the skirts opened to reveal warriors hidden inside – an African take on the Trojan horse.

Then, in the capital, Lomé, I went to the fetish market and bought a voodoo charm. The transaction was completed in a small dark shed, accompanied by a convoluted, bell-ringing, cash-extracting ritual performed by a "doctor" with a coconut shell. At the end of it I was the owner of a primitive little clay head with a fetching lock of blonde cow hair sprouting from its scalp. It represents the spirit Legba, and is supposed to protect a house against "the thiefman", but only if it is supplied with one cigarette a year or a drop or two of alcohol.

The fetish market is not there for souvenir collectors. It is a gruesome pharmacy for traditional medicine. The whole open-air compound is a scrapyard of animal parts, permeated by the faint smell of dead wildlife. There were shrivelled chameleons for good luck and stuffed owls on sticks to prevent bad luck; hedgehogs for virility, shrunken monkey heads to cure amnesia and parrot heads to treat the vocal chords. There were pieces of rats, cattle and cobras, stones for pain relief, shells for success and ebony seeds to improve memory.

West Africa is one of the most complex and least illuminated quarters of the Dark Continent, one of the least visited by holidaymakers too. It is a quite different Africa from the countries on the opposite coast. There, in the more homogenised and gentrified lands of beach and safari holidays, Africa has been partially processed. West Africa is raw Africa. These are travellers' lands; for the novice and the curious a cruise is the perfect introduction.

As for Saga, few brands are more instantly recognised and more invariably misconceived. The name is not an acronym, but if it were it should be something on the lines of, "Serious Action for Genteel Adults" or, less decorously, "Sod Ageing, Get Adventuring". The Saga cruise I took had about as much to do with Zimmer frames as Club 18-30 with Schubert lieder. These were passengers for whom exploration was every bit as important as relaxation. They were undeterred by equatorial temperatures and the lack of air conditioning in rattling tour buses.

The West African cruise is not run every year. Mine was a month long, starting and finishing in Southampton. Not many people can afford to spend 30 days at sea, so the ship was less than three quarters full.

Next year similar itineraries are being offered in two separate cruises on Quest for Adventure, the latest addition to the Saga fleet, and Saga Ruby. In January Quest sails from Tema in Ghana to Cape Town; two months later Saga Ruby returns to Southampton from her 2013 world cruise via Cape Town. Among the countries the cruises include are Namibia, Ghana, Sierra Leone, São Tomé, Togo, Benin and Angola. Now that is adventurous.

At 24,492 tons, Saga Ruby specialises in long voyages. She is a sort of spaniel of the seas, compact, faithful, good-natured and one of a traditional breed. Launched in 1973 as the Vistafjord, she was the last passenger ship to be built on the Tyne. She belongs to an age when ships looked like ships, not inner-city housing projects. Not for nothing has she been described as a mini QE2. When Saga bought and refitted her in 2004 she was owned by Cunard and called Caronia.

Her looks are classic: sharp, clipper bow and dark-blue hull, topped with a swooping sheer line. Curving white superstructure is banked beneath a tall yellow funnel. The Mayans built temples on much the same lines.

There are other indicators of her design era. Her decks are teak, there is a dedicated cinema, the indoor swimming pool is larger than the one outdoors and she draws 27ft of water. In fact the draft of this 660-berth ship, with nine passenger decks, is exactly the same as that of the ill-fated Costa Concordia, which carried 3,700 passengers and had 13 passenger decks.

Ruby's bathrooms are tiled rather than marbled and her walls are panelled in wood, not laminates. By today's standards the 35 cabins with balconies are relatively few and some of the smaller staircases are a bit like back stairs at Downton Abbey.

There is a lot of Downton about the ship. Not in any upstairs-downstairs sense – in English class terms it is more Downton Close, a doughty enclave of the middle class – but in the nostalgia on which this style of cruising thrives. The ship's library is a good deal longer on military history than literature and the unfailingly splendid afternoon tea was as ritualistic as voodoo. At 4.15 every afternoon a horseshoe-shaped table in the middle of the ballroom was stripped of its sandwiches, cakes, canapés, pastries and scones with the efficiency of Agent Orange. Occasionally we were expected to wear evening dress for dinner, just as they did in the old days. Even the best views from deck were looking backwards, over the stern, to where we had come from.

Every passenger I met was not just happy but proud to be on an older ship. They saw it as upholding the time-honoured values of seafaring, which in their view are British anyway. They call themselves Saganauts, and for them ships such as Ruby represent one of the last outposts of civility in the face of the advancing vulgarity of today's mega-ships with their thousands of passengers and the attributes of cities. "Supermarket trolleys, we call them," sniffed one septuagenarian who had repaired for a heart-starter to the five-star terrace of Reids Hotel in Madeira, Ruby's first landfall.

They are a generation who wear plimsolls or gym shoes – not trainers, which are for horses – and to whom competition is not a word tainted by 21st-century misgivings. They thrived on contest and still do. There were competitions in darts and shuffle board, golf and Strictly Come Dancing, quizzes and table tennis, deck quoits, "build-a-boat", whist, Scrabble and bridge.

There were kippers for breakfast, ice cream on tap round the clock, entertainment every night, lectures on days at sea and gentlemen hosts to dance with ladies without partners. There were lunchtime grills on the open deck and dinners of up to six courses in the dining room. They were served in a single sitting, but you were allocated tables. If you booked, you could eat in the much smaller View Restaurant for no extra charge. I heard no one complain about the food or the jolly service from the Filipino staff. The only mutterings were to do with the expense of some of the shore excursions.

Throughout the cruise there were three recurring themes. One, right down the coast, was the anticipation of oil wealth. São Tomé was typical: the whole place was on tenterhooks waiting for oil. Houses have been built at today's bargain prices and mothballed for tomorrow's boom. The banks – an inordinate number for such a small and impoverished country – are poised for petróleo's payday.

Second was the resignation to corruption. Wherever we went there was someone – a guide, a driver, a travel agent – who repeated, almost verbatim, "This is a rich country, but we are poor." The third theme was in the pitiless relics of slavery, the manacles and chains, the clenched dungeons, airless and satanic, the awful statistics. At least 12 million slaves were shipped to the Americas from these shameful shores.

What was disorientating, disturbing even, was to come across places of atrocity that are now so deceptively pretty. Île de Gorée, a notorious slave-trading post, in Senegal looks like a bijou village on the Côte d'Azur. Cape Coast Castle, one of the great trading forts that stud the coast of Ghana, could be a monastery, with its blue shutters, whitewashed walls and rolling courtyards. It is as if the grotesque apparatus of the Holocaust had been installed in prim cottages. But West Africa is as rich in hope as it is in oil. My guide in Ghana was christened Justice.