This sounds like a photosensitivity reaction. Photosensitivity reactions may range from solar urticaria to more severe reactions to UV rays. In your case, this may be solar urticaria. “Solar urticaria is an uncommon disorder sometimes difficult to treat. It is characterized by the occurrence of typical whealing reactions on exposed skin a few minutes after sun exposure. The reactions resolve 1 to 5 hours after sun exposure ceases.”
This type of photosensitivity may present with rash, hives and itchiness on sun exposed skin. Taking oral antihistamines prior to sun exposure may help. However in severe forms, you may need desensitization therapy. Applying sunscreen may help unless this also aggravates the flares.
Does the rash resolve in a few hours?
Within a week, the blotchiness areas will come about the 2nd or 3rd day of being in the sun and will remain a few days after the week is over. I usually put regular lotion on the areas to moisturize them so they wont itch so much, this helps some. I've also learned of something called Tinea Versicolor and have seen pictures and my case looks very similar to that. However, I am not certain because that disorder doesn't say much about being in the sun, just that is caused be hot and humid weather - which I guess being at the beach would be hot and humid?
This year I think I will put a high SPF on my chest and stay out of the sun. What a bummer, I will be in Aruba!!!