Hi there
I´m in Barcelona at the moment and, funnily enough, went down to one of the beaches today. There are about 4, all with a different atmosphere, which are all perfectly adequate. My only addition to the above suggestion would be to walk there, rather than catch the metro, along the harbour front as its quite magical - especially when the sun is shining, the catalan people are dancing the sardana and live music is drifting from Barcelonetta. Aaah!!! I don´t want to go home.
Have a great stay in Barcelona and remember to have a couple of glasses of cava at the dirt cheap bar on C/Reina Cristina at Barcelonetta.
Honest Goose