Hope that all goes well with Charlies tests. I did not know the answer to your question...found this website that says no food or water:
Preparing a dog for a bile acid test | VCA Animal Hospitals
This one does not mention water:
http://petloversvet.com/link/pat_hea...mBileAcids.pdf
Hopefully someone will see this and have the right answer. Calling an EC as someone suggested might be a good idea.