This one’s easy.
Once you’re done with exploring Colonia, head southwest (about a 10min walk from the old city) towards the ferry dock along Rivera Ave. You’ll see the ferry station there; it’s quite obvious:
It’s also very nice inside. There’s a check-in counter immediately in front of you when you enter, so get your ticket there (or if you prepaid online, get a new copy of an official ticket). Afterwards change your currency at the exchange counter nearby and proceed to Immigrations, where they’ll stamp you out of Uruguay and into Argentina at the same time.
Afterwards, head upstairs to the waiting room where there’s free wifi for you to use.
Boarding begins about 10min before departure, so line up quickly; I found out the hard way that seats run out pretty fast and the boats are usually sold out to capacity.
Once you find a seat, sit back and try not to get seasick. The ride, especially at the beginning, gets pretty rocky.
There’s also a big duty free shop in the center of the boat if you get bored (or hungry). Things do slip and fall when the boat lurches, so don’t be surprised if you hear glass clunking (or shattering) every now and then.
After about an hour on the water, you’re in Argentina!
If you checked in baggage, be prepared for a long (30+ min) wait as they really take their time with baggage claims.
Take a look outside the window while walking towards Buenos Aires:
– At time of posting in Buenos Aires, it was 69.8 °F – Humidity: 64% | Wind Speed: 15km/hr | Cloud Cover: scattered clouds