(Page 106 of the red book)
I’m not sure I can answer your question about how to pick a question from a statement, except by context, but I read this as a question.
Jiji, hearing the forecast - Listen, they say the weather’s going to deteriorate
Kiki, not making any attempt to care - What, when it’s this beautiful? Look, you can see the sea. Aren’t there heaps of people stripped off and having fun? It looks like the weather forecast is wrong. (etc)
I pictured a bunch of people still in the water as well as on the beach, with 裸 meaning “wearing less than their regular clothes” which fits with Kiki’s embarrassment at the idea of being seen in such a skimpy outfit when Osono-san first gives her the swimsuit.