Uruguay is worth a long stay. Give Punta del Este a miss but Piriapolis is a good place to start. Montevideo not the best of places to visit by yacht but an easy bus ride from Colonia. Colonia itself is a beautiful and very historic little town and a good place to base your yacht. From there you can get a long way up the Rio Uruguay which has a number of good ports
on the Uruguay side.
Personally I would give BA by yacht a miss....the Aduana can make your life a misery... but there is a good fast ferry service
out of Colonia.