I have a physical allergy called cold urticaria. I know with mine it's important to stop the exposure as soon as symptoms (hives) worsen - that means warming up as fast as possible. Occasionally I take Benedryl as a preventative. I've strange skin to begin with, I call it "silly putty" skin because I mark up so easily. I've also gotten hives on my arms just from the pressure of carrying something. I'm not usually itchy in water but any temperature other than tepid (cold or hot) and I'm bright red and blotchy head to toe.
I'm sure you've tried tons of stuff already, bear with me as I go through the list - an emollient? A barrier between your skin and the water, like baby oil or aloe? Capsaisin cream? Sodium bicarb in the bath water? Low flow shower head - I know I can't use the super strong hotel showers, I end up looking like a murder victim
. Allergy meds like benedryl or stronger? Ultraviolet-B phototherapy is new and better for your skin than all-over tanning. I assume you've been tested for illnesses that can cause aquagenic pruritis like Polycythemia and iron deficiency? All cotton clothes, sheets and towels are a must for me.
I've read they're trying Naltrexone (Vivitrol, ReVia), it's actually an anti-drinking drug for alcoholics but I guess it effects pruritis too. They're already using it in folks with Cholestatic Pruritis. (gallbladder/liver disease)