At the end of the Ramblas, where the trees turn to palms and Barcelona meets the sea, Christopher Columbus stands atop an enormous cigar, his arm outstretched, apparently gesturing towards the New World.

It was in Barcelona that Columbus was first received by the Spanish royalty, who commissioned his exploring in the name of Spain. Even further back, Barcelona was an ancient Roman port, then called Barcino. It covered an area now occupied by the Barri Gótic, a district of atmospheric streets and winding back alleys showcased in Woody Allen's 2008 film Vicky Cristina Barcelona.

Find your way out of this labyrinth and you'll be within sight of the monument to the great Italian. And thanks to a 20-hour ferry link, and the possibilities of cheap flights to Barcelona and from Rome, you can combine these two great cities with ease.

From his perch, Columbus actually points almost due south, across the water towards Algeria rather than North America. But however inaccurate the explorer's gesture, the sentiment is there for all to see: rather than this being the end of the line, the sea is the start of the next big adventure.

For centuries of travellers before – and for me on this sunny winter's day – the next stop is Rome. In a time when rapid and stressful air travel is the norm, I want to take my time in reaching the Eternal City.

Barcelona's harbour is lined with pretty bars, ganged by patio heaters to keep off the worst of the evening chill. I signal to the waiter as I drain my glass and she asks why I can't stay for another. I gesture to my ferry, visible now across the water by its blue lamps and bright cabin windows: I'm taking that boat to Italy in 40 minutes.

I say my goodbyes to one of Spain's most-visited cities and I head towards the sea terminal, on my way to the exact equivalent in Italy. After a brief check-in, I board the boat through the pedestrian entrance, while passengers with cars are herded into the main hold.

I arrive in my four-berth cabin – my ticket/keycard working on the fifth attempt – where I find my two room-mates chatting conspiratorially on the lower bunks. We quickly give up on conversation, realising we share neither language nor interests. So I do as Columbus did, and scan the sea from the top deck as we power east across the Mediterranean, the lights of the Catalan coast disappearing into darkness.

The passengers on board are a varied bunch. Loved-up tourists on rapid European jaunts walk the decks, searching for an appropriately exposed railing for a photoshoot. Truck drivers abound, making up at least half of the clientele. They have their own seating area, the unambiguously named "Drivers' Lounge". A violently decorated room, it stayed empty for the entire trip (perhaps due to the lack of a bar), save for a pair of teenagers sneaking a fumble in a garishly lit corner. After an outrageously priced Coke in the bar and some people-watching from behind my novel, I return to the cabin and clamber into my berth. Sleep is easy after a garbled goodnight to my companions below, who continue to plot long after I drift off.

Ascending to the deck the next morning, any cobwebs are blown away by the scene outside. The Mediterranean is a deep blue, the ship leaving a path of brilliant turquoise in its wake. Most passengers sleep in, rising at about noon for the visual highlight of the voyage: the approach to Sardinia and Corsica. The place to watch from is the top-deck bar. The rocky coastlines of both islands appear impenetrable from a distance, but the gap widens as the ship veers north, making its careful progress through the passage. My mobile phone signal returns briefly, and those whose phones announce undelivered messages reply hastily before reception is once again lost as we progress onwards to mainland Italy.

As the afternoon idles away, the open-air deck becomes crowded with card players. Tourist couples and teenagers chat happily among themselves. The windows looking out to sea are wide and passengers gather alongside them, taking in the vast horizon safe from the wintry sea breeze. A grizzled man, sitting alone in a woollen polo-necked jumper, smokes lavishly by the salt-flecked glass, savouring the sea-scented tobacco fumes and looking irritated by the salsa music blasting from behind the bar.

Our destination is Civitavecchia, the sea port that serves Rome now that ancient Ostia has long silted up. It announces itself in much the same way that Barcelona departed, a long twinkling coastline, lights sparkling through the darkness. Disembarking is a simple affair, followed by a seafront stroll to the railway station.

From Civitavecchia, trains make regular departures for Rome (€4.50 single). We eventually shudder out of the station, the carriage empty save for lone individuals sleeping in the comfortable chairs. The line runs alongside the coast, which is identifiable only by the square rows of lights from other ships – from Sicily and Malta, Tangier and Tunis perhaps, from where passenger ferries also run – punctuating the gathering night.

As the train nears Rome it turns inland and all goes dark outside until the outskirts of the capital appear. Roman aqueducts and residential blocks grow taller as cars, trams and scooters surge below. Pulling into Roma Termini, where the train line stops abruptly, it feels not like the end of the line, but rather the start of the next one.

Travel essentials

Getting there

Grimaldi Lines (00 39 081 496 444; grimaldi-lines.com/en) sails from Barcelona to Civitavecchia for €45 per person in a regular seat, rising to €175 in an inside cabin.

Among the many flight options, British Airways (0844 493 0787; ba.com), easyJet (0843 104 5000; easyJet.com) and Ryanair (0871 246 0000; ryanair.com) serve both Barcelona and Rome from a range of airports.

More information

Barcelona Tourism: 00 34 932 853 834; barcelonaturisme.com