Seasickness is not a cause for concern on the average river crossing – but the River Plate is no average river. It took my fast ferry 75 minutes to get from Buenos Aires in Argentina to the Uruguayan town of Colonia del Sacramento on the opposite bank; on the day I'd chosen to travel this was at least an hour too long.
Argentina's poets have seen many different colours in the river – blue-grey, copper, metallic green – but I can confirm that the huge waves, whipped up by a wind from the South Atlantic were a particularly nasty shade of brown and any tinges of green were confined to the faces of the passengers.
It was not an ideal prelude to a visit to Uruguay's most popular tourist attraction. The town's mixture of Spanish and Portuguese colonial architecture stems from its chequered early history. Founded in 1680 by the Portuguese, for the next 150 years Colonia was on the front line of a battle between Iberia's two great imperial powers. It changed hands by conquest or treaty on 10 separate occasions before finally becoming part of the independent state of Uruguay in 1828.
Sixteen years ago, Colonia's "Historic Quarter" was added to Unesco's list of World Heritage Sites. Most of its two million annual visitors arrive, like me, from the Argentinian capital. As a cross-border day-trip, it beats Dover-Calais.
As you enter the Barrio Histórico through the city gate, convincingly rebuilt in its 17th-century form, that travel cliché of "stepping back in time" is difficult to resist. In front of you is the Plaza Mayor, originally a military parade ground but now a spacious, grassy square lined with plane trees. Beyond it a maze of cobbled streets stretches over a dozen blocks, bounded on three sides by the waters of the River Plate.
I set out on a haphazard wander, trying to spot the different cultural ingredients that have created Colonia. The evocatively named Calle de los Suspiros ("Street of the Sighs") is a typical Portuguese street from the town's earliest period, with a drainage channel that runs down the middle, unlike those constructed by the Spanish, which have pavements and drainage channels at the sides. The single-storey houses with tiled roofs, painted in pink, blue and other pastel washes are unmistakably Portuguese, while many of the larger two-storey mansions were added by the Spanish.
The largest building on the Plaza Mayor is the Casa del Almirante Brown; Admiral Brown was the Irish-born founder of the Argentinian navy. His house is now the venue for the Municipal Museum with its collection of furniture and weapons from the colonial era as well as an interesting scale model of the original town. It is one of seven small museums dotted around Colonia, all of which can be visited with a single ticket, costing 50 Uruguayan pesos (£2). Each has a different angle on colonial life: the Museo de los Azulejos has some fine examples of decorated ceramic tiles. I also found a handful of shops, selling attractive textiles and jewellery at not-very-attractive prices; among the blessedly few cars to be found on the streets were a number of 1950s American roadsters, enhancing the sensation that I'd undergone time travel or wandered accidentally onto a film set.
Around lunchtime, the sun came out for the first time and with it appeared a galaxy of restaurant tables, chairs and menus of the day. I eschewed the set meals and opted for tapas accompanied by live guitar music in the eccentric El Drugstore, whose walls are covered in posters and local art. A huge Spanish omelette, accompanied by a vast salad and a glass of earthy Uruguayan Tannat wine set me back 270 pesos (£10) and set me up for a sortie back into the real world on a rented bicycle.
"Unremarkable but not unpleasant" describes the modern town of Colonia, where most of the 23,000 inhabitants live, work and shop. A 5km ride up the coast – technically the river bank – brings you to a salubrious suburb of villas with gardens boasting substantial brick-built barbecues for the essential weekend feast of grilled meat. This area, near a river beach, is being developed for tourism. But the most surprising feature is a huge, gaunt, derelict bullring, which fell into disuse in 1912 after only eight corridas, when the Uruguayan government banned bullfighting.
Back in the Barrio Histórico, there was just time to walk along the ramparts, visit the Basílica del Santísimo Sacramento – a simple and much-restored white church – and then climb the narrow and spiralling steps of the lighthouse, which rises, impressively from the ruins of a convent and is the highest point in Colonia.
As the sun began to set, the River Plate gained some startling gold and silver bands, and the wind increased in strength, causing the lighthouse and me to tremble slightly. I had a bet with myself that my return ferry to Buenos Aires would be delayed – and it was, by some hours.
I was content to wait until things had calmed down a bit before my return river crossing. After all, one wall of the Shipwrecks Museum was filled by an alarmingly long list of ships that had sunk to the bottom of the Plate.
Travel essentials: Colonia
* Ferries between Buenos Aires and Colonia are operated by Buquebus (00 54 11 4316 6500; buquebus.com), Seacat (00 54 11 4314 5100; seacatcolonia.com) and Colonia Express (00 54 11 4317 4100; coloniaexpress.com). One-day returns start at 180 Argentinian pesos (£30) per person.
* From Montevideo, the bus journey takes two and-a-half hours (00 598 452 1990; turil.com.uy) and costs 196 Uruguayan pesos (£6.20) each way.
* Posada Plaza Mayor, Calle del Comercio 111 (00 598 4522 3193; posadaplazamayor.com). B&B starts at US$140 (£93).
* El Drugstore, Calle Portugal 174 (00 598 4522 5241).
* Casa del Almirante Brown, Calle del Comercio 77 (00 598 452 27031).
* Museo de los Azulejos, Calle Misiones de los Tapes 104 (00 598 452 21065).
* Museo de los Naufragios, Roger Balet y Calle de los Argentinos.
* Bicycle rental: Thrifty Car Rental, General Flores 172 (00 598 4522 22939); US$3 (£2) per hour.
* Uruguay Tourism: turismo.gub.uy