We took the 40 minute 'Eco Tour' trip in July 2013 - what a bargain at 7.50 euros! (For those with children, each adult can take 2 children for free!)
The boat departs from Port Vell and travels out and around the marina area. The boat has two 'nets' which you can sit on and watch the watch underneath. We were able to see numerous cruise ships in dock on the tour, including the Disney Cruise ship.
A very enjoyable and excellent value for money activity!