Recently completed an 11 day Buenos Aires to Rio de Janiero sailing on Seven Seas Mariner, our second Regent cruise having done Voyager last year.
We were semi-independent so arrived at the port at Buenos Aires, boarding and check in was as easy as our first RSSC cruise, with registration taking place on board which is MUCH more civilized (we passed a snaking queue of MSC cruisers standing outside the terminal in 28* heat)
The ship and all the services were excellent, we personally preferred Mariner to Voyager. We were personally introduced to our stewards who did as much as possible to help and keep us tidy.