Maybe just a respiratory infection??? She has no obvious obstructions looking in her mouth but her feet are not all all warm, nor is her bill for that matter
Respiratory infections aren't that common with waterfowl, I would be more inclined to think either a blockage of sorts, or that she inhaled water (Pneumonia). At this point, the latter seems most likely to me.
But, I'm not a vet, nor an expert. So take what I say with a grain salt.