This might be a chicken/egg type question, however...

Yet more two cents worth...I'd say you possibly need to gather up some chutzpah and go ask the experts, that is WA Opera, or whatever they're called. They may well say that some form of dance is a good idea, the days of an opera singer just standing and belting out their arias are gone with the wind (you can thank Joan for that, she insisted on running and signing in the mad scene in Lucia). However, the swimming is obviously helping keep her fit and she's definitely learning coordination so that's got to be good. Having worked in London, no it's not glamourous, just darn hard work, on the fringe circuit the one's who do best have had some training in all areas. Personally I am an actor/singer/dancer, in that order. To make it to the top in musical theatre there you must be a singer/dancer/actor, equal strengths. Of course there are exceptions, naturals who have very little training in one area BUT they are exceptions, so why hinder yourself? She's nine, there's still time for her to decide what she wants to do. But I would say that if the opportunity arises to do just one class a week in a general dance style it would be well worth her while, no matter if she goes for opera or MT. On the other hand, she might chuck the whole thing and become an astronaut! Did you watch Operatunity? If not, buy/rent the series. It is well worth watching and worth noting that the finalists nearly all had training, even if it was self taught. Note also the types of classes they had to undergo, including movement, fitness, dance, acting, etc. The natural bass won, mostly because, and I quote "he's a bassinet" but "you don't find these growing under trees". Rapunzel. "Papa, where is Mama? They tell me she has gone away, where is she?"