Across the Río de la Plata

Two ferry companies share the same Puerto Madero terminal in Buenos Aires, crossing the Río de la Plata — the wide estuary Montevideo sits on. On those sailings the bike travels free: officially so on Colonia Express, and by every traveller's account on Buquebús, though Buquebús doesn't spell the policy out in its published terms. A third operator, a smaller one, takes a northern route out of Tigre.

Buquebús runs three ways over. The direct Buenos Aires–Montevideo crossing is the Francisco, a fast LNG catamaran that makes it in a little over two hours. The Buenos Aires–Colonia route — shorter, about an hour and a quarter, several sailings a day — runs on the Atlantic III and the Silvia Ana L, from where a connecting bus finishes the trip to Montevideo in around four and a half hours all told. Fares span a wide range depending on class and how far ahead you book, so confirm the current price at the time. A fourth boat is coming: the China Zorrilla, an electric ferry that's been announced but isn't yet in service.

Colonia Express is the other operator, with about five crossings a day, and it's the one whose bike policy is written down plainly: one bicycle per passenger, free, provided you sign a liability waiver at check-in. Buquebús's own policy is the less formal of the two — the "free" is better attested in travellers' accounts and promotional pages than in a definitive tariff — so take the front wheel off, carry your panniers into the cabin, and confirm at check-in.

Líneas Delta Argentino works the northern route: its boats leave from Tigre rather than Puerto Madero and reach two smaller Uruguayan river ports. Carmelo is the busier of the two — out Wednesday, Friday, Saturday and Sunday at 08:30, back at 15:00 on Monday, Wednesday and Friday and at 17:30 on Sunday — while Nueva Palmira runs only Tuesdays and Thursdays, 08:30 over and 15:00 back. The crossing takes about three hours. Fares are in Argentine pesos and change often, so check the current price on the operator's own site before you rely on it; the company also sets out no bicycle policy, its terms covering only general luggage, so confirm a bike when you book.

The formalities happen before you board: at the Puerto Madero terminal both Argentine and Uruguayan customs clear you, so you step off in Montevideo already through — the procedure from Tigre may differ, so check when you book. On the way back, expect Argentine customs to fit a seal to a new-looking bike — one of the small frictions that come with the overland routes from Argentina, where the river is a border rather than a ferry ride.

Where you land shapes your first hour. Colonia del Sacramento is a small, flat, cobbled UNESCO town — lovely to wander, though its historic core is for walking rather than riding — and Montevideo from there is either a bus or a two-to-three-day ride down the coast. Land in Montevideo instead and there's no transfer at all: you roll off onto the rambla, the waterfront road that carries the city's cycle route. Carmelo, the Tigre boat's port, drops you straight into the wine country and its bodega roads.

Which to take. If the bike's certainty is what matters most, book Colonia Express. It's the only operator that puts the bike in writing: one per passenger, free, with a waiver at check-in. Book ahead and leave time at the terminal, and Colonia's cobblestones are a little over an hour's crossing away.