To me it really sounds like the culprit is the H2O molecule itself, degranulating mastytes and setting off a reaction or a toxic response. These people are allergic to rain, sweat, blood, tears, humidity - even fog. Aquagenic Uticaria or Aquagenic Puritis is diagnosed by placing tap water, then distilled water and observing the reaction.
Lemonade, diet coke and milk are safe as the water is carbonated or bonded into another compound; making it entirely different altogether - they can survive off of this. A single drop could send a young British girl into shock, she can only drink milk due to it's chemical structure and has to wear a special oil to stop water making contact with her skin, she cannot drink it as it would cause her throat to close up. She has to eat dry foods.
SOURCES: Medical journals, Opera, Tyra, Maury, 1,000 ways to die (woman died because the fire sprinklers went off, and she was allergic to water) news articles, talking to nurses/ friends and personal research.
3 of my friends experience feelings of their throat closing up or swelling when they drink bottled or spring water.
also, the girl in the UK has to run air conditioners constantly so she doesn't sweat, she cant touch her boyfriend if he's been sweating. For these people, a single drop makes their whole body break out into painful, burning itchy hives. More severe symptoms can occur and capascin cream can help. Life-time antihistamines.