If I had the chance to go back, I would take the more expensive Buque bus ( http://www.buquebus.com/cache/HomeARG.html ) as the Colonia express ferry was cheaper but smaller and, above all it was late on both ways, going and coming back, which means losing time specially if you are doing a day trip as I was.
The fare for an hour journey from Buenos Aires to Colonia was about 100pesos per way. If you want to have a seat make you sure you get there early and reserve yourself a seat or you will be made to seat the steps. There is a sundry shop as well as a cafe inside. When you arrive in Colonia, this is where the nightmare starts, there isn't a proper conveyor belt to retrieve your baggage, unless you know how to shout in Spanish, it was quite an experience for someone who has just arrive in South America less than 5 hours...!!! If you want it fast, make sure you have some tips with you...!!!! No food are allowed into Uruguay, baggage will be check. I have to say it is not very organise but fun in a way.
We were almost convinced to take a tour fromour hotel for a cost of $65.00.Instead we caught the local bus and for us this turned out to be the best option.
The bus station in Montevideo is new and very modern...with many different bus companies offering travel all over Uraguay and beyond.
We bought our retuen ticket from Turel,departing at 10:00 and returning at 6PM.It's an easy 2.5 hour trip .
The easiest way to reach Colonia from Buenos Aires is by ferry. A fast boat takes about 50 minutes and there is a slow boat, less expensive, that needs about 3 hours to cross Rio de la Plata river. I found the fast boat more suitable since I only made a day trip to Colonia. I checked in both offices directly, Buquebus and Colonia Express (a new company), and decided for the latter. Colonia Express offered better timetable and prices. But you better check before your trip as they change frequently.
I bought the ticket a couple of days in advance, directly in the office on Avenida Cordoba 753. It is open from Monday to Friday 9am-7pm and Saturday 9am-1pm. Depends on the season and the day of your travel, but early booking is always advisable. Be sure to have your passport with you when buying your ticket and checking in (Australians and Canadians also require a visa for Uruguay) and arrive at least half an hour before departure the scheduled departure time.
Ferry lives from Terminal Internacional de Cruceros, Ramon Castillo/Avenida de los Inmigrantes (phone number 011 4312 5050). I took the 8am ferry with the return from Colonia at 4:45 (3:45 Uruguayan time) and payed 179 pesos Argentinos. The ferry has Duty Free Shop, self-service bar, very comfortable seats and TV. My trip was on the day before Christmas and we were only about 10 people on board. Passengers were offered coffee in the morning and we left the ferry with Feliz Navidad, having Christmas dinner already in mind :)
Ways of getting to Colonia other than Ferry
I did find the ferry very convenient and easy, but you may also consider other routes to get to this nice town. Most tourists though arrive via the Rio de Plata ferries.
You can drive through bridges to get to Colonia - From Buenos Aires, by bridges Puentes Zárate-Brazo Largo and Internacional Libertador General San Martín.
Highway #1 connects Colonia to Montevideo and points east; Highway #21 connects to points north, including the Aarón de Anchorena National Park, 30 kilometres (19 mi) distant, and Fray Bentos.
The Aeropuerto Internacional de Colonia Laguna de los Patos is located 6 km from the city center, but it is just for smaller airplanes and helicopters.
I decided to ride the boat from Buenos Aires to Colonia del Sacramento, Uruguay and it was really worth it. I rode in the morning (not forgetting my passport of course because I am heading international) and then went back in the afternoon. If you want to check out the times for the website, check it out at http://www.buquebus.com/cache/HomeENG.html
They also have two different kinds of boat, depending on speed and class (fast or slow). The fast boat will take you there in about 50 minutes, whereas the cheaper slower one takes about 3 hours. Being someone who wants to see the most, I chose the faster one.
I have also read that there is also the Ferrylineas hydrofoil which leaves from Darsena Norte to Colonia, and also arrives in just 45 minutes. Colonia Express and Seacat Colonia are other companies offering ferry rides.
But in any way yo go to Colonia, you will enjoy walking around this beautiful town in Uruguay.
The times and prices have changed and so I will just refer you to their website.
We travelled to Colonia del Sacramento from Buenos Aires on the Buquebus ferry. The crossing to Colonia was fine and the ferry was as nice as any we've travelled on. There were more than enough seats for everyone with a good number right beside the windows allowing you a great view. There is an especially impressive view of Buenos Aires when you leave the harbour.
We took the cheapest ferry which not surprisingly was also the slowest taking 3 hours to cross the Rio Plata. There are faster ferries which do the crossing in an hour and these are a good option if you plan a short trip to Colonia. You can book your tickets at the ferry port or at one of the many Buquebus offices in BA.
if buque bus is sold out (we tried getting our tix on a friday for a sunday trip and no dice), try colonia express, a new ferry company in BA. the prices were the same. their office in ba is just a few buildings down the street.
Like almost all others, I took the Buquebus boat from BA to the port of Colonia. The boat made it there in about 50 minutes. The Buquebus port is at Northern Dock in Puerto Madera.
I bought my tickets 2 days in advance, but if you go during the busier tourist season, or on a weekend, you should book them farther in advance. I went on a monday (May 21), bought them on the previous saturday. Some of the times you want might be sold out, or at that time the only boat available might be the 3 hour boat....so book as soon as you get to BA, or on the internet.
I bought the ticket at a travel agency that sells them, that are on practically every other block in BA, especially in the little malls. The cost was about 150 pesos ($50) r/t. You will need your passport number to purchase them, so bring it. All exit fees are included in the price of the ticket.
For my 8:30am trip I got to the station at 7:30, and had a little time to spare, but it wasn't high tourist season. and DON'T FORGET YOUR PASSPORT! You ARE going to another country. And bring a pen, you'll need to fill out a few forms of course.
To catch the bus from Colonia, when you get off the ferry you walk out and stay towards your right - you will see a small bus terminal where you can purchase tickets.
From Montevideo you go to the bus station which is behind the mall called Tres Cruces (you have to go the lower level of the mall at the back).
The company I went with was COT - it took about 3 hours and they picked up passengers throughout the whole ride.
Buquebus - (the ferry company) also offers bus transportation to and fro - you can check their website for exact prices for both the bus and ferry costs.
The cost is approximately 160 Uruguayan pesos which is about $5USD.
There are a few options to get to Uruguay from Buenos Aires. Least appealing way is taking a bus directly there from Retiro. Most popular option is to take a ferry. There are 2 options for the ferry - the slow ferry (3 hours to Colonia) and 1 hour.
Check the Buquebus website for current prices
It's also a good idea to buy your tickets to Colonia/Montevideo in advance during peak season as it gets quite busy. You can buy them online or go to Buquebus in Buenos Aires (not too far from Retiro bus station).
I can't begin to reave as much as I would like about the freey service, it was only bad in BA at least. The people in BA are great at making the ship depart 1 1/2 hours late while everyone stands in a hot room and waits wondering what is going on.
When going to Colonia from Buenos Aires have patience at the ferry station the ferry always leaves late because the people running the operation don't know what they are doing. I was suppost to leave at 9 and left at 10:30 the boat was there and we all stood for over an hour waiting on the company to decide we could board.
If you are approaching Colonia and Uruguay as a matter of fact from Buenos Aires, there is no other reasonable way exept to use the ferry service by Buquebus. The employees of this company realize their quazzi monopoly position and take full advantage of it. They gave me quite a bit of hassle when I had bought a return ticket but due to bad weather the boat could not leave. When I tried to recover my money they were resorting to all sorts of ways to persuade me that this was not possible. Thanks to the havens, finally I was reimboursed and later on bought a passage again.
About 89P/$30USD per person, one way on the hydrofoil (fast boat). Took 55 minutes.
Buses available at the port when the boat arrives to take you on to Montevideo.
Bus station is on Av Roosevelt (4 lanes) a few hundred yards outside of the port gate.
A ticket on COT from Colonia to Montevideo cost 153P/$6USD per person and the trip took us a little less than 2 hours.