A sarong with a stretchy belt sewn in is a lifesaver to be able to cover your lower legs and feet if you get sunburned (or preferably to keep you from getting there, makes a decent towel, good head cover for when you are sleeping outside and it is either windy or buggy, allows you to get out of your shorts and let things air out, makes a nice ground blanket for quick catnaps on the sand, get it wet and lay it over the cooler for evaporative cooling ....
The list goes on. I never go boating without one and I put one in each of the kids personal day-time dry bags along with snacks, sunscreen, splash jacket, etc. I can't think of a trip where I don't have a picture of someone crashed out on the stack with a sarong wrapped around them. Even when dry they cool you off.
I bought my mother in law a UPF 50 one for our Salmon trip last year, from Prana I think. It was a bit spendy but not crazy. My favorite is a Rayon one I got in a parking lot at a show over 20 years ago. It literally just gets softer and more comfortable with age.