Hello, and welcome!
Has your cat been diagnosed with any underlying medical condition, like chronic kidney failure, which is causing the dehydration?
The typical fluid used in giving subcutaneous fluids to cats, is Lactated Ringers Solution. I would not used distilled water to hydrate your cat subcutaneously.
If she is severely dehydrated, besides watering down her baby food (make sure this doesn't contain any onion or garlic products), she needs to be evaluated in person by the vet, blood work is necessary, and once a diagnosis is made, treatment will be given and/or prescribed.
If you haven't brought her to the vet due to financial circumstances, the following may help you:
The following vet office provides a coupon for a free first visit, so you might want to use try this, if finances are tight right now:
You might also consider applying for Care Credit which is accepted by most vet offices:
This is a list of organizations that might help pay for vet visits:
You can also contact your local Humane Society, ASPCA and cat rescues to find out which vets they use, that may be lower-cost and/or may see your furbaby, gratis.
I hope all will be well with your kitty!
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