There is a bus and train service into the centre of barcelona train was free when i used it last year bus 3 euros taxi from the train station or cataluna plaza bus stop about10 or 12 euros . there is also a new train station at the bottom of la rambla 100yds to bus stop will take you to the ship. search barcelona tourism. it is really easy but dont buy anything on la rambla and watch you bag/wallet. have a nice trip.
It's only about 15/20 mins by cab. The cabs arrive by the dozen along side the ship, just join the queue.