Try zooming around on this map:
www.acme.com/jef/hotsprings/

As he says, some of the data is out of date but it can still be a helpful guide.

I just got back from a trip to Harbin Hot Springs near Middletown - had a wonderful time. Secluded location, yes, but it's going to be crowded this season. Harbin's a resort - not sure if that's what you're looking for. You pay a $30 fee for a membership (only one member of your party needs this) and $35 per person on weekends for a 24 hour visit, including camping if you want (no fires allowed, but you can pitch a tent or sleep under the stars). Info at www.harbin.org.

If you're looking for more of a crick for a strip 'n' dip, this ain't it. I'm aware that there's a spring near Stinson Beach, but I understand it's hard to get to and you have to judge the tides just right. There's a <a href = "post</a>" title="soakersbible.tribe.net/thread...</a>">soakersbible.tribe.net/thread...</a> about this - probably more info elsewhere...

Good luck!

harbin hot springs or saratoga springs. Both are resorts/training facilities.

There are others up in those same mountains, they're geothermically active. But I don't have any idea what's available, accessible, safe, open, etc.