A person with Aquagenic Urticaria is essentially allergic to water. It is an extremely rare condition that affects mostly women and can develop at puberty. Its rarity means it's often misdiagnosed as an allergy to chemicals in water, such as chlorine or salt.
Symptoms include severe skin itching after coming into contact with water - or blood, sweat and tears - as painful, itchy hives spread over the affected area. These can last anywhere between 10 minutes to an hour-and-a-half and some sufferers have to lie down to let the pain subside. Swelling of the throat can occur after drinking water.
There is no proven treatment, so the recommendation is to avoid all forms of water, keeping showers short and avoiding rain.
There is limited data on successful treatments, but for some patients, antihistamines — which are often used to counteract allergic reactions — help. Some other medications, light therapy, and sodium bicarbonate have been found to be helpful as well.
More allergy information and advice here