What do i need to build an japanese onsen?what do i need to start an japanese onsen buisness in the us?

Question:

anything you have to say is welcome
and yes it is the traditional naked bathing thing where you sometimes just strip in public and it is sometimes mixed sexes

Answer:

Yes, as all the posters point out, in Japan, you cannot use the term "onsen" unless it is natural hot spring.
Finding and digging hot spring from 2,000 meters underground takes an enormous amount money in Japan.
But in the US, I guess you don't have that law, do you?
I guess you can use tap water to do your "onsen" business in the US.
You can name it Ukiyoe Onsen, Geisha Onsen, Ninja Onsen or whatever.