I have a mild form of this, as does my daughter. It’s not a case of being allergic to water, but to sweat. A hot shower, exercise or sitting in the sun can cause a breakout of terrible hives, mostly on the back of the neck and upper arms. Scratching or even just touching them makes it much worse. Benadryl or other allergy meds can help, but then you feel like a zombie for hours!
Maybe Singulair (or its generic) would be beneficial?