Outdoor rash - can anyone help?

Hello, can someone help me out with my daughter's rash? 16 years old, generally fit and healthy and on a combined contraceptive pill for dysmenorrhea. She is getting these itchy rashes/urticaria (see photo) whenever doing activities like swimming in rivers, springs or the sea. Even sometimes when it is raining and cold outside, but not when she showers. We have tried antihistamines but it doesn't help. She has also had a sun allergy since childhood. Is it some sort of allergic reaction? Are the two conditions linked? Anything to give her for symptomatic relief?