Hi and welcome!
If you want another professional's opinion, write to Dr. Nelson via http://birthmark.org/experts.php
However, I've always heard that sun exposure will worsen PWS and most doctors and patients will recommend sunscreen at least. Of course, that is true for most everyone now.
My kids both have swim shirts for the pool or the beach. These are those long sleeved, thin, ultraviolet resistant shirts and they LOVE them. If you google "spf shirt" you'll find lots of places selling them.
My opinion is to err on the side of caution. If you can afford a sleeve or shirt, why not? Since lots of kids are wearing them now, at least she won't be teased.