sorry to read that. Insurance companies are swine. Although , morally, they should accept this as reason to cancel they may say that unless doctor states she is unfit to travel then ,sorry, this is not a reason to cancel even though she can't swim.
I may be wrong but best to plan for the worst
So, don't tell them that she may be OK to fly but not swim. Ask for a claims form but don't give too many details when you ask for it. You must speak to your doctor and get a nice letter from him stating that he cannot recommend that your daughter should travel. And if he won't give it then make sure your daughter screams in agony at the next visit.