There is a bathroom and lockers under the volcano. Not sure if they check ID to enter the pool. They might if it looks to capacity, but I'd ask at check-in.
Sorry, from your post, I thought you were going to be a guest at the Polynesian, but after re-reading your post, not sure now. If you are a guest, my info still applies. But if you aren't staying at the Polynesian, it isn't right that you swim there because unless you are DVC you have no pool hopping privileges.