Is Marbella, Spain a nice place for a basic tourist/swimming holiday?

I plan on going to Marbella for 10 days in August for a seaside vacation. Nevertheless, some friends of mine who have already been there have given it quite negative reviews, stating that muggy, non-translucent water and dirty sand ruins the sunbathing/swimming fun. Is this really true (honestly)? Or is Marbella actually as pretty as the pictures I have seen (quite beautiful)?