The onsens are WONDERFUL and one of the reasons I'm itching to get back to Japan. I never had a second thought about the nudity, even though they *were* watching us because we were the only white men in there. Whatever, just a penis, we've all got 'em.
The hot wasn't too hot for me but I admit I had trouble in the Denki Buro "electrified" pools: a low current is positioned on one end and you walk from the steps at the other end, about ten feet away, towards it.
You only feel a little tingle at first, but as you get closer it amplifies and your muscles begin to contract, your heart pounds, your throat constricts, you feel the hair rising all over your body — and then there's actual pain. I couldn't go more than three feet, but some of the Japanese gentlemen casually swam from the steps right to the other side! How'd they do that?!