Buquebus is a transporation company that offers ferry/bus services between Argentina and Uruguay. The Buenos Aires station is in Retiro, near where Cafe Orleans is located. It's not too bad of a walk if you do not have a lot of luggage.

Their website is: http://www.buquebus.com/

When you visit the website, you are presented with four options for home country, but there are really only two versions of the site at this time, the Argentina and Uruguay versions. The site is only offered in Spanish despite the Brazilian and English flag options.

There are many route options throughout the day, but it's best to buy your ticket at least one day in advance to ensure availability.

On my trip last August, it cost me approximately 1400 pesos for roundtrip service between Buenos Aires and Montevideo. At the current blue market rate that's about $140 US. The ferry only took me to Colonia, I had to ride the rest of the way on a bus. The direct ferry was 500 pesos more.

The ferry and bus were both very comfortable and I wouldn't hesitate on taking them again.

The station in Montevideo is called Tres Cruces. It's a shopping mall with the bus terminal underneath.
http://www.trescruces.com.uy/

If you need US dollars, you can withdraw them at the ATMs in Montevideo, or at least in the bus terminal, but I think all Montevideo ATMs dispense dollars.