Being on the suncoast of FL for nearly 40 years, most visitors are not prepared for the intensity of the sun here, even in the winter. One visitor to my aunt's, had a severe burn after many hours on the beach. Poison control said to put her on a plastic sheet or shower curtain and slather her with plain yogurt. It took the 'heat' out of her body and saved her a trip to the ER.
But when someone over does when visiting us, we apply aloe on a sunburn. Usually it does the trick and keeps the blisters at bay if you can use it immediately and apply from time to time. The aloe is from our aloe plants, some of the blades are 3" wide and our front flower garden is full of them. A plastic bag full of blades accompanies them home and sometimes the whole plant. Every home needs some!
I keep a pot of them on the kitchen window ledge, great for any kitchen burn, garage nick, bug bites and more.