What Diego said.
Uruguay must be the most laid back country on the planet.
I hung out in the Rio De La Plata from mid 2009 until the end of 2010.... Colonia is a brilliant spot at all times of the year while Piriapolis is good for haulout and working on your boat but more of a holiday town and not as much fun in winter or the off season imho.
You can go quite some way up the Rio Uruguay, I believe as far as Fray Bentos if not further.
Based on my experiences with the Aduana and sundry other Argentine thieves I would avoid BA and the delta
like the plague.... as suggested take the fast ferry
out of Colonia if you want to go sightseeing. Or take a slow ferry
out of Nueva Palmira through the delta
First 2 pix.... Colonia, 2nd pair Nueva Palmira and the slow boat through the delta to BA.