I flew on a miles ticket in J in N0vember, and the homeward routing was MVD-EZE-ATL-BOS. The MVD-EZE was on an Aerolineas commuter partner (with an Aerolineas flight number). You might be able to find a fare that includes the final partner leg to MVD. While the ferry is pretty fast, EZE is located quite far from the harbor so you will spend a fair amount of time getting to the ferry. Some of the ferries go directly to MVD but most ferries from Buenos Aires go to Colonia and connect to a bus which takes you to MVD (about an hour or so on the bus).