It is true that the water is not very clean, but as it a big-city beach, so it's absurd to expect crystal clear water. What makes it special is not the water, it is the incredible atmosphere you can feel. The good thing about it is that it is very accessible, since it is a few minutes away from the metro station.
If you want a more dynamic atmosphere I recommend going during the high season, when there are more activities to do and more people. I like going during the rest of the year, since it is ideal to relax with the sound of the sea and to have a walk.