Had another thought -- does the cruise ship take your luggage from the exit airport all the way to the ship & reverse that on the way home? If not, or if you want to verify that your luggage did make it there, or if you are doing your own air...
PACK LIGHT!
Don't know about Barcelona, but the custom lines in Madrid were long. And, as in any European airport, you don't want to let go of your luggage at any point for any reason.