We have always had good experiences with taxis in Barcelona. We’ve used them on four visits, airport to hotel pre- and post-cruise, hotel to cruise port, cruise port to hotel, and also within Barcelona to get to some of the sightseeing locations.
If you have time in Barcelona before your cruise, I agree with the above poster to see Sagrada Familia. We did a self-guided tour from one of the Rick Steves guidebooks to Barcelona, purchasing admission tickets to see the interior online in advance. We have also enjoyed the hop-on/hop-off bus tour. That can be done purchasing tickets at the time you want to do it.