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Hi :) There are couple of hotsprings that you can swim inside the Park.

But I imagine you are asking about that rainbow colored lake , and sadly no , you can't swim there. Actually color is like that because of the sulfur oxidizers, so its not safe

For the river I posted; I do not know If you can swim there, it was pretty cold when I visited the place. I would say still no. There are some dedicated swimming areas and it did not looked like one