I've only been with Paperback Swap for a little over a year.  Today I received a book that had been exposed to water. The pages aren't simply wavy, they are so curved that the book barely closes all the way.  I emailed the sender, but have yet to hear back.  Is there anything else I can do, if I don't hear back from them?

Did you ask for your credit back?  If they do not respond in 7-10 days, use the feedback button at the bottom and report them to R and team.  Hopefully you marked the book RWP as well

I clicked RWP and told them about the condition of the book.  I didn't ask for a credit back yet, I guess I was just too nice and waiting for an explanation as to why it was in such a condition. 

Here's my recommendation:

1. Examine the wrapping. Did the book get wet in transit? If so, mark RWAP, Damaged by USPS. Inform the sender about what happened and suggest using something more water-resistant in the future.
2. If the wrapping is not water damaged, mark RWAP - damaged by Sender. PM the person stating that the book is water-damaged and does not meet basic PBS posting guidelines. Request your credit back (if desired) and request a response within 7 days.
3. If sender responds, discuss issue with sender and see if you can reach resolution. If yes, go to archives and select "problem resolved". Sometimes, even with amicable discussion you might just need to agree to disagree. In those cases I would select the "Unable to reach agreement" (or however it's phrased).
4. If day 7 passes, and you've received no response, you can report to R&R using the "Contact Us" link. If you're convinced that there will be no resolution (rude response, for example), you can mark the problem "unresolved" in your archives. I believe that serves the same purpose as manually reporting the member.

If a member is particularly rude, offensive or threatening, report the member right away using the "Contact Us" button.

If you want your credit back, you need to ask for it.  A lot won't refund the credit if you don't specifically ask for it.

If I determine the book was damaged before the other person sent it (and not by the PO), I write them a PM explaining the damage I see on the book and requesting my credit back.  That way they know what the problem is and what I'd like them to do to resolve it and the ball's in their court to either refund the credit or not.