The mannerism of the pose will sort this better than correct posture, so as it is suppossed to be a teen character, i would, like miss nancy said, have her arms wrapped around herself, for two reasons, one it would work better if she looked a bit cold, and secondly, it would look better also if she didn't feel so comfortable "naked". 

To sell the youthfullness of the figure to the viewer, she should look a bit awkward with her nakedness, maybe not so far as to be trying to completely hide herself from prying eyes, but enough so that it looks like she is aware that someone could/might see her, if you incorporate that into the pose, it will overwrite the incorrectness of the pose, by giving off a message to the viewer (us) via her body language of a timid approach to the waters edge, as oppossed to the viewer being left to just judge the correctness of the figures balance.

Anyway hope that made some sense, and helps you out.