People flock to Barcelona for its amazing beaches, some of which are in the city and others that lie on the outskirts. What's so great about this town, is that the beaches are so close to the city center, so you can easily visit a museum and then hop on down and take a dip in the sea. Depending on what you're looking for there is something for all tastes, even a few naked beaches for those who are bathing suit averse.
A few particular hot spots include in front of the W Hotel, where the water is a bit cleaner and where surfers tend to congregate in the early mornings, as well as a bit farther out beyond 'Gherry's fish', to beaches such as Bogatell and Marbella. These are only a short walk up the coast and often are less touristy and less crowded with excellent chiringuitos (beachside huts).
Regardless of where you go though, the best times of day to hit up the beaches is in the morning. Although you'll probably spend time with the 'golden oldies', the water is calm and clean and the beach is relatively empty. There's also great water sports to explore including paddle boarding and surfing as well as sea-dooing and jazz cruises.