What kind of water is she getting....city, spring, well.....what kind of dish.....is there anything that could be especially attractive. Certainly if it is behavioral, a behaviorist makes sense. However, I still believe there is something physical (either with the cat's condition or with the water or dish itself) that is causing this. And, while I am glad to hear and respect that you have had her checked by two vets, there are still very unusual circumstances that the average vet isn't familiar with. I hate to be stubborn, but let me ask you this.....think of humans.....you know many humans who over or under eat because of psychological issues. However, have you ever known a human who drank too much water because of one? I just think the answer is missed to this point, but believe there is an explanation. From a behavior standpoint, I have never known a cat to drink too much water....many have been known to drink too little. Hopefully, someone will show up who has seen this before.
I would also experiment with her food. Maybe there is something in it that is driving her to drink. For instance, Fancy Feast is quite salty.....I can imagine a cat that didn't like the salty residual taste and kept drinking to get rid of it. I would look at all possible variables that might be changed or tried.