We booked this cruise at the last minute at the suggestion of a friend who knew that we were still in Barcelona. We booked an inside guarantee and received an upgrade to an outside room (Booked Z, received I).
Embarkation port in Barcelona is at the commercial port of Barcelona. While the closest metro station is Drassanes, I highly suggest a taxi cab as it is quite far from the metro station. (Note, taxis must go to the end of the road and turn back, so don't freak out when they go past the ship for the first time). The embarkation took longer than it should, mainly because they were using one scanner for all the passengers (they eventually opened a second scanner when the line was well out the door.) They had a separate are for Platinum/Diamond and Suites at the front, with the rest going to a central area. Another area was cordoned off for those with a set-sail pass. We knew that the ship had not sold-out and enquired about upgrades and we were told that none were available. (We enquired later at the customer service desk only to be told that we should have asked outside.)
A note about cabins on the 2nd level of the Voyager, the floor is split in two and you can't pass from fore to aft on the 2nd floor. You need to walk through the Royal Promenade on the 5th or through the Casino Royale on the 4th.