i'm so sorry that happened. why oh why didn't the doctor put on a waterproof cast??? i don't know what kind of break she had or if there was a specific reason why she couldn't have that type. did you go to an ortho place, or was it just your regular doctor's office? i don't know what your financial situation is, but if you can swing the cost, maybe you should check into getting it recasted into one that can go into water. those do exist--we've had them. it's just that you're paying SO MUCH for your vacation, a few hundred more so she can swim might be worth it.
of course, if it's not possible to change the cast, she will still have an awesome time!!!! we've taken lots of trip where we didn't bother to swim because you can do that anywhere--we stuck to disney only things....