Rash which appears after contact with water can be due to aquagenic urticaria. Water urticaria, or aquagenic urticaria, is a rare condition in which hives develop within 1 to 15 minutes after contact with water. The hives last for 10 to 120 minutes and do not seem to be caused by histamine release like the other physical hives.
It is diagnosed by applying tap water and distilled water to the skin and observing the reaction. Contact dermatitis can be diagnosed from your symptoms and physical examination. Blood tests and x-rays are not helpful.
Treatment of water urticaria includes epinephrine, antihistamines, or cortisone-like drugs. Itching can be controlled with cold packs or commercial topical agents that contain menthol, camphor, eucalyptus oil, aloe, antihistamines, or cortisone preparations.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.