It wouldn’t be biologically possible to be “allergic” to water. It would be like being allergic to carbon. You can only be allergic to proteins or things attached to proteins (haptens) since they trigger an antibody response.

One interesting story about faulty immune triggers is in the book “Brain On Fire” where she had horrible brain inflammation and psychosis. After a long hard struggle trying to diagnosis it they finally figured out her immune system had attacked an ovarian cancer but surface proteins on the cancer cells matched receptors in her brain cells so her immune system was attacking those too.