It has happened before, of course. Venice's early invaders were Goths and Huns, Byzantine armies and Lombards. But, by the late 13th Century, it was the most prosperous city in all of Europe. A major maritime power, too, home to 3000 merchant ships all armed in case of attack, and carrying passengers who were also armed and required to fight when necessary.
Venice's new invaders, however, are a glossy well-fed race favouring the voyage of the river cruiser that is fast becoming the Laguna Veneta's new armada. The river cruisers are long, slim craft - almost as elegant as gondolas themselves - and present a pleasing scale to both passenger and destination when compared with the infernal mega-liner cruise ships that daily logjam the Giudecca Canal and blot out the Venetian Gothic majesty of the city's skyline.
Venice is just one European destination witnessing a revolution in the cruise industry, with river cruising, the industry's fastest-growing segment, chalking up passenger increases of 10 per cent every year for the past five years. Cruises on Europe's waterways such as the Rhine, the Moselle, the Danube, the Rhône, the Seine, the Volga and the Douro are proving the most popular, but even the Yangtze and the Mekong have joined the international river-cruise map.
Already there are confirmed river-cruise matelots with six and seven rivers behind them, and they have spoken. What they like is seeing destinations that can't be reached by ocean cruisers, docking in the heart of river cities, guided tours included in the price of the cruise, small-ship congeniality, seeing countryside and cities from an unusual perspective, and the fact that, as an emerging concept, there's the attraction of doing something different. Add to all of that the extreme unlikelihood of sea-sickness.
For river-cruise operators, the race is on to impress with new destinations and itineraries. Uniworld's Boutique River Cruise Collection has added Italy's longest river, the Po, to its program with an eight-day cruise embarking and disembarking in Venice. The company has been careful to promote this not as a great maritime odyssey but more a floating hotel experience, because there is little more than 20 hours' sailing time in the eight-day program. But there is no denying the appeal of an itinerary that reveals the beauty and mystery of some of Italy's most fascinating cities - Venice, Padua, Ravenna, Verona and Bologna.
Uniworld's River Countess turns out to be an impressive boat built in 2003 and totally remodelled in 2012. With four suites and 63 staterooms across five different categories, she can accommodate 132 guests and provides enough public areas in which to find privacy or conviviality at will.
Décor is Regency-meets-casino but the overall impression is lavish - a powerful magnet to the river-cruise passenger. Uniworld talks the talk when it promises "Turquoise complimented [sic] by cream, taupe, and white are used throughout the ship to create a luxurious and sophisticated environment for you to experience Italy's dolce vita".
The chandelier is a recurring design feature but ameliorated by an excellent collection of artworks.
We may be ploughing along the Laguna Veneta but there, on a wall of the Rhine Deck, is the comforting familiarity of Sir Sidney Nolan's The Watch Tower.
Rhine Deck staterooms and suites offer the advantage of a French balcony, but all rooms have panoramic windows that make them light and airy, as well as affording expansive views. The Savoir hotel-style beds are dressed in Egyptian cotton sheets and a European quilt, and come with a pillow menu. There are built-in closets with drawers and plenty of hangers, and unpacking just once while getting to visit five fascinating cities has to be another big plus for river cruising. All staterooms have a flat-screen television with infotainment service and satellite link, and individual air-conditioning controls. The marble bathrooms feature luxurious towels and robes, a hair dryer and L'Occitane bath and body products.
While the stateroom accommodation is pleasant, if a tad chintzy, River Countess offers a wide choice of public areas and thoughtful amenities to keep passengers circulating socially and experiencing new dimensions of the, how-we-say, dolce vita.
The Il Castillo Lounge, venue for cocktail parties, lectures, evening entertainment and all-day chilling, offers a full-service bar, as does the more intimate Captain's Lounge and Library. The Sienna Salon Sky Lounge provides casual dining, and there are also opportunities to dine and enjoy bar service and snacks on the huge rooftop sundeck. Other features include a spa, fitness centre and sauna, laundry, 24-hour coffee and tea station, and a computer room with web access.
The main dining room is the Savoy Restaurant, well-organised and large enough to serve the ship's entire complement at a single sitting. Food is important to river-cruise passengers, many of whom get their entire week's exercise circling the buffet before launching repeated lightning raids on the rum-soaked almond cake with whipped cream and chocolate sprinkles.
The food is generous and most days runs through a gamut of early-riser breakfast, sumptuous breakfast, late breakfast, lunch, pizza and salads, snacks, cocktail canapés and dinner. Dinner offers a selection of antipasti, soup and main course dishes plus desserts and cheeses. The extensive menu shows a commendable intention to remind diners they are in Italy, but in deference to the age and palates of the passengers the food is more Lifestyle Food telly than Locatelli.
In case that's too harsh, executive chef Roberto Bettolini did wow us with a simple but superb dish of spaghetti with laguna mussels and clams in a parsley, garlic and tomato sugo. For those in denial of being in Italy, there's an alternative menu of geographically non-specific "safety net" dishes, plus both vegetarian and light-and-healthy menus for those yet to reach the summit of the food chain.
But for all the comfort and conviviality of life on board, the shore excursions are what will remain etched in the memories of those who sail the Po. For the first three days of the itinerary, River Countess is moored at various docking stations around Venice, which allows plenty of time for guided tours and independent ramblings of the city and its major attractions. Uniworld appears to have managed to get first choice of local guides who, throughout the itinerary, proved first class.
The highlight of the guided tours around Venice was a boat ride along the Grand Canal at dusk ending at Piazza San Marco for an exclusive evening tour of St Mark's Basilica, with an expert lecture that gave a stimulating insight into the paradoxes of religion, politics and wealth that Italy has managed to juggle throughout its history. If the architecture of the basilica is Byzantine, its history of power struggles is positively Machiavellian.
Other highlights include a guided tour of the Doge's Palace, for centuries the heart of political life and public administration of the Venetian Republic until the empire's fall in 1797. Everything in the palace is on an epic scale, including the famed artworks of Tintoretto and Veronese. Tintoretto also heads the bill in the Scuola Grande di San Rocco, an optional excursion, as is a visit to the Tessitura Bevilacqua, which explores the art of silk weaving in Venice since 1700.
For those who don't mind a bit of harmless humiliation, there's even a chance to take a class at gondolier school. This does not include singing, as the serenading gondolier is more a thing of the past, something that would cause Gilbert and Sullivan to turn in their respective graves. For tourist-amusement workers, the gondoliers are surprisingly serious about the preservation of Venetian culture and eventually mutinied over being asked to sing "O Sole Mio" and "That's Amore" by the culturally confused. If you require serenading, the gondola company will find a vocalist to share your boat, but he belongs to the entertainment union, not the gondolier's association, and you pay him separately.
This misconception about singing gondoliers still creates the occasional frisson of excitement, though. When a mature North American passenger asked her gondolier if he was going to sing, he looked at her with those toxic Latin eyes and said, "There are lovers and there are singers… and I do not sing".
There was a guided tour, too, of the inevitable tourist traps - the Piazza and the Rialto - but with as many as 130,000 tourists a day (40,000 more than the local Venetian population) packed into the city on peak days, the charm of these areas is sorely challenged. This is the Venice of the fridge magnet, the plastic gondolier's hat, the "I heart Venice" T-shirt. Better to drop off the back of the tour group and find some elbow room - and perspective - around Zattere, the Giardini gardens to the east, or the Via Garibaldi, all populated by real Venetians rather than tourist hordes.
Best of all, seek refuge at Harry's Bar at the end of Calle Vallaresso, which somehow captures the spirit of La Serenissima with just a pinch of Hemingway. For me, a visit to Venice without a visit to Harry's Bar is unthinkable. This famous but resolutely un-hip place has been above and beyond the infidelity of fashion since Giuseppe Cipriani opened it in the 1930s, and it is hard to believe it has changed at all. This is where Cipriani created his famous Bellini cocktail (Prosecco di Conegliano-Valdobbiadene with white peach) and his carpaccio appetiser (finely sliced raw beef dressed with lemon, oil and shavings of parmesan). Ever since it has been visited by local society, visiting celebrities, Ernest Hemingway and impecunious travel writers.
From docking points at Serravalle and Polesella on the Po river and Chioggia on the lagoon south of Venice, cruise passengers are able to take coach transfers of between one and two hours for day tours of Padua, Ravenna, Bologna and Verona. While Venice, Padua and Bologna titillate the visitor with some of history's most classical romances, Verona gets to carry the baggage of Shakespeare's Romeo and Juliet.
The fact-or-fiction debate surrounding this story is still very much alive, with fiction just a bit in front. But in Verona, guides solemnly shepherd tour groups to stand under Juliet's balcony, just where Romeo stood, and point in the direction of Juliet's burial site.
Best, I think, to reserve your awe for the wonderful Arena, the third largest and best-preserved Roman amphitheatre in Italy. Verona is also home to Vinitaly, the world's largest wine expo, built on the heritage of Valpolicella. Uniworld passengers are offered an optional tour of the vineyard country and the Serego Alighieri wine estate where local ripasso, Amarone and recioto wines are paired with regional delicacies.
Ravenna promises to be a lively compact city with a large student population. We visit on a public holiday though, so while the city centre is in shutdown, the basilicas, baptisteries and belltowers are buzzing.
My right knee has long forgotten how to genuflect, but Ravenna's glories ignite a prayer candle deep within. Ravenna was a capital city on three occasions: capital of the Western Roman Empire; capital to Theodoric the Great, king of the Ostrogoths; and capital of Byzantine Italy. Its basilicas, the Basilica of Sant' Apollinare Nuovo and the Basilica of San Vitale preserve some of the richest mosaics in the world, some dating back to the 5th and 6th centuries.
Padua is to frescoes what Ravenna is to mosaics, and at Scrovegni Chapel we admire walls and ceilings covered in frescoes by Giotto. The impressive Basilica of St Anthony, which attracts some four million pilgrims every year, is also on the itinerary, and our prayers are answered with lunch at a city restaurant with a generous appreciation of Veneto's regional wines: Bardolino, Valpolicella and Soave. Despite its pleasant human scale, Padua - a city of some 210,000 residents - features the biggest square in Italy. Prato della Valle, once a vast open-air theatre, is today surrounded by a canal (installed to solve earlier flooding problems) and adorned by 78 statues of Padua's rich and powerful.
Bologna is the food capital of Emilia-Romagna, which, one could argue, is the food capital of northern Italy. This region takes gastronomy so seriously that it promotes 15 dedicated Streets of Wine and Flavours, an organised circuit of wine, cheese and prosciutto producers, bakers, cellars, delicatessens and restaurants. Museums here don't have sculptures of people with missing heads and arms. Among 19 Museums of Taste, there's the Museo del Prosciutto di Parma, the Museo del Parmigiano-Reggiano and the Museo del Balsamico Tradizionale. And when Emilia-Romagna has its Wine Food Festival, it lasts three months.
Officially, the two leaning towers of the Asinelli and Garisenda families in the centre of the city are the emblems of Bologna, but what really characterises the city are the arcades and markets in the historic city centre. By the way, spaghetti Bolognese is a contradiction in terms. Here, in its heartland, the dish is always made with tagliatelle, never spaghetti. On their visit to Bologna, Uniworld passengers can participate in a pasta-making workshop at Cantina Bentivoglio and enjoy their new artistry in the form of tortellini in brodo.
Every one of the sophisticated group of River Countess passengers I spoke with was impressed by the onboard dining. But for me - and the small group of food adventurers I was kindly recruited into - my favourite aspect of river cruising was the ability to eat authentic regional specialties every day in some of the great food cities of the world. Try doing that on a cruise ship.