Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
Now in regard to your question, we do need to tread with care for Rosie. Birds can become dehydrated very quickly which can make them feel poorly. In her case, b/c its watery we have to consider not just GI issues (eg infection, etc) but also kidney troubles as well. And b/c of that, I would lean to having her seen as soon as your local vet can do so. That way we get to the heart of this and address it before it can take a toll on her.
Any delay and for the moment, there are some steps you can take. To start, you do want to make sure she keeps eating and drinking well. The latter is especially important to counter diarrhea losses. If that declines at any point, we may need to hand feed and tempt her to take both. For fluids, tempt with fruit juice flavoured water or an electrolyte solution (ie Vit-al). If her appetite lapses at any point, you could use nutrient paste (eg Nutrical) to supplement her.
For loose stool, check your local pet store for an avian probiotic (ie Avipro plus, Benebac,etc) as this will support her gut and help normalize the gut bacteria present. And if she is very runny, then you can try giving 1-2 ml of OTC Kaolin/Kaopectate to slow the diarrhea. Though again b/c watery stools can mean either body system, if we do not see this settling with the side we can treat, then she is best seeing the local vet once they are open. They can assess her stool, send it for testing if needed and ensure we target treatment as best we can for Rosie.
Just in case you do need an avian vet and do not have one already, you can check where you can find one at near you at AAV (http://www.aav.org/search/), Avian web(http://www.beautyofbirds.com/recommendedvets.htm) or Birdsnway(http://www.birdsnways.com/birds/vets.htm).