Yellowstone Park, of course, has a super abundance of thermal features. However, be advised that most of them will cook you like corned beef in a couple of minutes. Besides, you will probably fall through the thin crust before you get to the feature and the boiling mud below will change your mind about getting any closer to that blue green beauty. And that is to say nothing about the gimlet-eyed guys and gals in green and grey who may pounce you from the brush. Be advised that it is highly illegal to do anything more than follow the board walks when looking at thermal features.
Hideous stories abound about accidents which happned to people who ignored the warnings.
However, there are two places where you can enjoy the water. The first is Boiling River, just north of Mammoth. Hot springs join the Gardner River there and it is a favorite spot on moonlit nights for lovers. Rumor has it that the place was Boiling River. Also, there are a couple of places near the Madison Campground, one on the Madison River and one on the Firehole River.