If you are traveling between Buenos Aires and Uruguay, even for a day trip, chances are that you will at least look at taking the ferry across the Río de la Plata. Dan & I have done it a few times, and its much easier than one would think. Its also much quicker than flying between countries considering the time getting to the airport, security, etc.
Buenos Aires (BA), Argentina is quite close to Uruguay, only separated by the Río de la Plata. As such, a common and convenient way to get between BA and Uruguay is to take the ferry, generally referred to as the “BuqueBus,” between the two countries.
In fact, ferries depart several times a day between BA and Colonia, Uruguay and BA and Montevideo, Uruguay and back. The journey to Colonia takes a little over an hour, while the journey to Montevideo takes around 3 hours.