Another vote for Barcelona Day Tours if that's in your plans (they were AMAZING) and/or just using a taxi if needed. We were on the Med cruise last August and flew in 2 days early (from Germany as we visited there first) and then another two days after the cruise. We used a taxi from the airport to our hotel (Hotel Jazz - another 5 stars for them too) and then taxi from hotel to the port (pretty close by), then again from cruise to hotel again, then hotel to airport. No issues at all.
This was also in the middle of the Las Ramblas terrorist attack (so devastating) and we felt it was the best for us.
Have a great time,
Heather