My brother, who is both a professional musician and the world's biggest music snob, has a pat answer he gives when asked what he thinks of a band he considers mediocre: "They're uh, good for what they are." Translated, this generally can be taken to mean that, while this particular band doesn't rise to the level he considers art, and they may not even be particularly talented, for the type of music they play, and the audience they're aiming for, they don't completely suck.
That pretty much sums up how I feel about Aquamarine. It's fluff, sure, it's certainly not as smart as other films aiming at its general target market (preteen and adolescent girls) - like Mean Girls, Freaky Friday, or even Ever After, a much better fairy tale film - but it's mostly forgettable and harmless fluff. It has the feel of a Disney or Nickelodeon film made for television, that somehow, inexplicably, made its way to the big screen instead.