Ditto with the above. Immigration authorities in South American countries that do not require a visa for entry have never, in my experience, asked for proof of onward travel. AA agents, however, routinely do so at check in. While they have previously accepted my word when I had an electronic ticket in their system for return travel, they have been much happier when I presented an actual printout of said ticket. So I assume they will be much happier with a printout of your Buquebus ticket out of Argentina (plus your return flight from Rio) than just your return ticket from Rio. Besides, Buquebus uses a variable pricing system similar to the airlines, so buying a ticket in advance is recommended in order to obtain the lowest price anyway.