When you look at getting a ferry to Montevideo (or Buenos Aires) there are two options. You can take the ferry to Colonia del Sacramento from Buenos Aires and then hop on a 2-hour bus (what many people do, as that's most of the time a bit cheaper) or you can take a ferry from Buenos right into the heart of Montevideo. (Or the other way around of course)
The normal boat takes about 5 hours, and the fast boat (actually the fastest ferry in the world!) takes about 3 hours.
Popular Tourist Destinations
Both Montevideo and Buenos Aires are popular tourist destinations, with plenty of things to see and do in each city. You are crossing a country border though, so you do need a passport to cross the border.
If you book this trip at Directferries, you will notice that the Colonia Express connection is a combo ticket. (A crossing + a bus)
Which ferry companies sail on this crossing?
How long does the Rio de la Plata crossing take?