Will this hat get ruined if it gets wet?

No. Hat has held up very well to the Florida afternoon showers.

I wear a 7-3/8" hat will this work?

I wear a 7-5/8" hat. A slight squeeze for me, but I still wear this for hours doing yardwork. for 7-3/8" I would expect this to be comfortable.

I put a measuring tape around my head, and I have a 24 inch head circumference. Will this hat work or stretch too thin ?

I have the same size head as you. NO, it will not be to thin. It’s an excellent hat for yard work or whenever you’re outside in the sun. Plenty of shade measured by the brim😎

Is it safe to assume this won't work for a 21.5" head?

Just for you I measured my head. I am 21.5. I love this hat. I genuinely want to protect my fact, neck and shoulders. I don't like sunscreen on my face specifically. This hat works. I love it.