The lock-keeper, grey-haired and in his fifties, strolled out of his ivy-covered cottage to close the gate behind our boat. Looking at his unhurried pace and scholarly mien, my companion took a guess: “He’s an old lettresstudent and he took the job to get back to the thesis on Nietzsche that he will never finish.”

Christine was only half joking. With its peace, still water and alleys of dark chestnut trees, the canal seems suited to a life of reflection. Yet we were in the heart of Paris, a few hundred yards from the roar of the Place de la République. A lifelong Parisian, Christine had never seen her home town like this, from a boat on the Canal Saint-Martin. “It’s like stepping out of time, yet very familiar,” she said.

That mood is easy to understand because the 2.5mile (4km) waterway, built in the early 19th century on the order of Napoleon Bonaparte, opens doors to a forgotten city. A few times a day, open-topped boats cruise for two and a half hours along its length, between the Bassin de la Villette and the Arsenal pool by the Bastille. Less spectacular than a motor along the Seine, the slow trip reveals an intimate, sometimes secret, Paris. Gliding along the canal, descending the eight locks to the Seine, you sense how the old city’s north-central quartiersmust have felt when life was briefer, but much slower. Like the catacombs and the cemetery of Père Lachaise, the canal journey brings constant reminders of mortality.

“On your left is la Maison des Morts,” says the loudspeaker as we slide by the elegant limestone house where they paid rewards for bodies fished out of the canal. Leaving the Écluse des Morts, the lock of the dead, we are reminded that near by was the site of the terrible Montfaucon gallows, a huge structure where the hanged were left to rot by the dozen.

Little is left of the once-bustling river traffic that supplied food and materials to the city’s heart using the waterways of the 18th and 19th centuries. The wharves are still in use at the broad Bassin de la Villette in the north. That is where the canal joins the city’s two other water thoroughfares, the Canal Saint-Denis and Canal de l’Ourcq. The Villette gives a sense of the old port life, like a small version of the Docklands in London.

Like London’s wharves, the gloomy waterside warehouses are being converted into trendy flats. Behind them lies the green expanse of the Parc de la Villette, developed by the late President Mitterrand with red steel sculptures and showcase “cities” of science and music. From the canal, though, everything looks different. Looming on the skyline are the Grands Moulins de Pantin. The castle-like Victorian-era flour mills that you flash by on the périphérique motor-way have a new grandeur when seen from the waterways that served them.

Just about everyone, including those who have never set foot in Paris, has an image of the Canal Saint-Martin, or at least its pretty stretches with their arching iron footbridges and boatman’s paths. In the early 1870s they inspired Alfred Sisley and fellow Impressionists. Most recently Amélie, the 2001 film on retro Paris, opened on the canal’s Écluse du Temple, with its swivelling road bridge. To France and film buffs everywhere, though, the canal is a legend as the location for one of the country’s best-loved films: Marcel Carné’s Hôtel du Nord.

When our boat – named Arletty after the star of the film – rounded a bend and emerged on the Quai des Jemmapes, recognition rippled through the passengers. In the gloom of a rainy August afternoon, there was the little hotel. We were back in the 1938 tale about young lovers who come to the hotel in a shabby part of town to kill themselves. To add to the ambience, loudspeakers played one of France’s most famous movie lines. “ Atmosphère, atmosphère. Est-ce que j’ai une gueule d’atmosphère?” (. . . does my mug look like atmosphere?). Arletty’s voice rasped with its old working-class twang.

The black and white façade of the original Hôtel du Nord was saved from demolition in 1984 by Arletty herself, and a bar restaurant, restored by a film-set designer, occupies the ground floor. Its tiled walls, brass fittings and chequered floor give it period charm. Its menu is modern brasserie, served by young staff. The cocktail this month is called Atmosphère.

Once the haunt of labourers and boat crews and near derelict by the 1960s, this section of the canal has been adopted by the young and the hip, les bobos (bourgeois-bohèmes), who are taking over working-class northeastern Paris. Prices have rocketed with the conversion of lofts and the appearance of galleries and design shops. Across the canal and a little lower is Prune, the most bobo of the Saint-Martin bistros. Last winter this stretch of canal became a long-running story when an organisation called Les Enfants de Don Quichotte erected more than 100 hiking tents for homeless men. The stunt, aimed at tweaking bourgeois consciences, ended when the city offered the men lodging in an army barracks.

It is hard to believe now, but in the early 1970s, President Pompidou considered approving a scheme to turn the run-down canal into a motorway that would bring traffic from the north into the heart of the city.

As if in a movie, the bright canal world, with children playing and men fishing under the plane trees, vanished as our boat edged into the darkness of the vault of the Temple. A few shafts of light from the boulevard above were the only illumination as the damp brick ceiling skimmed overhead for a mile. After 10 minutes in this dank netherworld we slowed to observe overhead the strangest sight of the journey.

Dim grey daylight illuminated a crypt. Here in silence lie the remains of the victims of the revolutions of 1830 and 1848, and straight above stands the column of the Place de la Bastille and all its traffic. Two minutes later, we emerged from underneath into the light of the Bassin de l’Arsenal, a mooring for pleasure craft and houseboats before the canal drops another lock and joins the Seine.

Emerging into modern Paris again, my friend forgot her blasé Left Bank instincts and confessed surprise. “ Quel dépaysement,” she said. English has no word to convey that sense of suddenly being in another, strange, country.

Read Charles Bremner's Paris blog at timesonline.co.uk/blogs

Where to stay
By Jane Knight

Laorana
Hire your own houseboat on the Seine, moored in front of the Jardins des Tuileries. The Laorana has two floors and two terraces – plenty of room for five people. Details: 00 33 147 05 15 94, http://timesonline.holiday-rentals.co.uk, property 51268; £1,696 a week.

Hôtel Saint Jacques
Head back in time to the Belle Epoque in this pretty Fifth Arrondissement hotel. Details: 44 07 45 47, www.hotel-saintjacques.com. Doubles from £68.

Hôtel du Petit Moulin
Christian Lacroix has created a riot of colour in the heart of the Marais. Expect grafitti murals and panels with Lacroix’s trompe l’oeil drawings. Details: 42 74 10 10, www.paris-hotel-petitmoulin.com; from £122.