Several questions. Where did she come from? Private owner, sale barn ????? Do you know if she got colostrum or just what you were giving her?
It could very well be coccidiosis as that will cause most scours in the 1-4 week time frame. It is supposed to be species specific; that said, I only have that problem when I keep calves at a barn where there were turkeys about 5+ years ago. And poultry are known to carry coccidiosis and often not have problems. Is there any blood in it?
How much water is she drinking? Honestly if she is drinking much water, she may just be getting more "liquid" than she needs right now. Plus, if it water that is from a spring or any type of access to water that is at all connected to run-off, or a stream or creek, there could be something in the water that is causing it. Even well water can cause some upset in very young calves if it has alot of limestone, or iron or sulfur or anything like that.
At 1.5 weeks, I think that she doesn't need so much of the water or calf starter. I never push mine to even try it before 2-4 weeks and usually they start it on their own from curiosity. If she were on her dam, she would be mouthing a little grass or hay but not getting any nutrition from it for a couple of months. Milk is what babies are designed to get a good start on. I realize milk replacer is expensive, but it is still the first best food for them; for a couple of months. That is why I just do not see the pushing them to get into "feed" before about 3-4 weeks. Getting them weaned by 8 weeks is what most shoot for, I like to wait to 10-12 weeks as they just seem to do better. If you are a top notch manager, like some on the dairy farms, you can get them weaned at 6-8 weeks. But you are putting in alot more expensive high quality feed to keep them on a "growing curve".
One other thing. Is the milk replacer - ALL MILK??? If there is a SOY base to it, that is most likely a big part of the problem. Calves do not digest any soy based milk replacers well. They might survive, but they will not thrive. If it says milk protein, whey , or anything like that it's okay. If it says anything with soy, it is not. And it should be a 20/20 protein/fat base. They do not need the 24% protein with only 16% fat. That is usually a soy based formula. You can get an all milk based replacer, and when the calf gets up to 6-7 weeks, use it half with the soy based to use up the soy if you don't want to waste it, which is what I have told many people in the past because it is very costly. By then, their gut tract can handle some other things and will not have the negative impact that it will as a new baby.