Note: if you have visible tattoos, you should not use an onsen. This is a pretty strict etiquette, so don’t try it on or you are likely to create offence.
What do they mean by visible tattoos? From what I understand, you have to be naked so any tattoo is pretty much visible then. They only "not visible" tattoo I can think of are if you have one inside your mouth? I'm just being picky, I think it's just a clumsy way of saying no tattoos.