I've just come back from Valencia and I spent 2 days at the beach - the one(s) to the north of the harbour. I was very impressed by the quality of both the beaches and the sea, which I swam in on both days. The water was clear, seemed very clean and, almost a week later, I have not yet developed any diseases or allergies and all my organs seem to be functioning as well as ever!
But seriously, it is a lovely strip of beach and the sea is really great for a dip - there was no seaweed, oil film, jellyfish, or any other rubbish floating about in/on it.
Another thing I liked about these beaches was that, unlike at Barcelona, there were no lowlife beachbums hanging around, and the ambience was really chilled. You can happily leave your stuff on the beach when you go for a swim and not have to worry about it (I certainly wouldn't have done that at Barca on my own!). Of course, I was there in the last week of Sept, and no doubt it gets a lot busier in July and August, but I'd be surprised if the type of people who go there would be any different, just more of them.