Hi, I can understand your concern for itchy red skin with hive like spots after swimming. I can get from query that you are suffering from Aquagenic Urticaria or water induced hives. It is a common condition caused by bursting of allergy producing cells known as mast cells due to certain aggravating factors present in the environment. As the chemical inside them is released it produces a rash which you have mentioned. As you know very well the best part is staying away from aggravating factor. But other than this the long term effective treatment for this is by oral anti histamine like loratadine under medical advise. It is one of the drug which is not having any side effects and can be taken for long term usage. You can take loratadine daily or preferably few hours before swimming. Take care.