Soaking in a hot spring seems unnecessarily risky. There's too much chance of substantial temperature change. Better to soak in a pool that's fed from a hot spring. Rather than mess with park police, find hot springs where it's allowed.
Locals know places where it is relatively safe to soak in the hot water. Heck, you can go to Chico and pay a few bucks there to soak in the same water at no risk and drink a beer..
