My DD is the same way. I just bought her a zip from (higher neck) tank suit, knowing that she will ALWAYS wear a rash guard and shorts with it.
If you check rei, they do have on piece short sleeve/shorts suits, but make sure your DD has enough support. A lot of the "rashguard" type suits have NO support, so while they cover, they are meant to be worn with regular suit underneath.
I also just ordered DD a long sleeve one piece from Title Nine. It was ridiculously expensive and has not arrived yet, but DD is excited to try it on.
We have had some luck with Lands End (but they are more "mom" styles) and Title Nine...
Good luck... you are not alone in your search!