Buquebus has frequent boats and boat+bus trips to Uruguay. It is possible to visit Colonia in a one day trip, the fast boat takes 1 hour for the ride. The boat is a large one that also carries cars, and at least in the month of March (and probably most of the time) it fills up a few days in advance so purchase your tickets a week or so in advance. Tickets can be purchased online at the web site, or at many of the Buquebus offices around the city. It is easier to purchase the ticket at the offices (or the web, but web requires registration) and there is one office at Ave Cordaba 879. The lines for purchase at the terminal were quite long and the purchase requires standing in two lines - first to book the ticket, and then to pay for it. In March 2008 the price for a single day round trip on the fast boat was AR$184, which is a discount from the multi-day return trip tickets. The Buquebus terminal is at the end of Ave. Cordoba - just keep walking on the street towards Ave. L N Alem, cross that street and the next few adjacent streets and the terminal will be visible quite easily. Passport (or at least the number and other details) is required to purchase the ticket.
In Colonia if you are only there for a day, you can use AR$. All restaurants and shops and musems accept these and they use the exact same rate as the Compra rate listed in the banks so there is no need to convert currencies, especially no reason to convert AR$ or US$ at the exchanges at the terminal which provide a poor exchange rate. Change, if any, will be given in Uruguyan currency, though.
Colonia Express is another boat service to Colonia, and it has a office at Cordoba too. They have a single trip out and return and they had a promotion of AR$198 for day trip that included a guided tour as well as lunch. Their terminal in Buenos Aires is around 2 kilometers from the BuqueBus terminal, I believe.
The process of immigration and customs can be a bit disconcerting, especially at the Uruguay end. In Colonia there was an extremely small room that had to have two lines - one for check-in and second for immigration and it was just crazy with winding lines and no directions on where to go first and which is the right line for check-in, and then for immigration. There were no rest-rooms in sight, and at least an hour can be spent easily standing in these lines.
They are constructing a new building at Colonia, so maybe this will improve in the future. Entry immigration for both directions for both countries is done at the departure point, so when you arrive, you can breeze past quickly with no standing in lines other than customs which is usually just a quick wave for day-tripping foreigners. Also keep the papers that are stamped in the passport - looks like in Argentina they stamp this additional sheet of paper that you must have when you try to get back into the country. Which does not make sense since no such paper was needed when I entered Argentina from the USA, so not sure why the Argentina immigration at the Colonia Uruguay departure point needed the paper stamped by Argentina immigration at the Buenos Aires departure point. Keep all stamped entry/exit documents easily accessible, is the lesson here. For those tourists looking to extend the 90 day permit they get when they enter Argentina, making a day-trip to Uruguay is an easy way to add another 90 days of stay in Argentina.
[March 2008]