This is coming from a lot of experience getting a lot of strange looks during many years living and faux pas-ing in Japan.
1. Don’t Think About Soup
I’ve brought non-Japanese friends to a very nice Sento, or public bathhouse, and heard the inevitable reference to “sitting in an old guy soup”. “The only thing missing is sliced carrot and celery.”
2. Don’t Dip Your Face Towel in the Water
In a sento or onsen, when walking around, a lot of people carry a small face towel with them. This towel is very convenient for covering private areas while walking around and mopping up sweat. It is not, however, to be dipped into the water.
I learned the above lesson the hard way when I was sitting in a very nice Onsen in Okayama pretending that my face towel was a jellyfish floating around in the water. I’ll never forget the horrified look on my friends faces. Come to think of it, little threads from the towels could could easily get into the water and if a lot of people did it, make the water look horrible.
This is an easy mistake to make because, for obvious reasons, people on travel shows in Japan bring the towel into the water with them to keep everything family friendly.
3. Don’t Forget to Go to the Bathroom Before Getting In
It could just be me, but I have a bad habit of getting into the bath area, washing up, and rushing to the comfort of the water. No sooner do I get in though then I remember I have to go to the bathroom.
A general rule of these places is that the bathroom is inconveniently located and they supply the most painful waterproof slippers they can find for use in the facilities. It’s always good to take care of all that before you start enjoying yourself.
4. Don’t Swim
You might seem kids and weird adults swim, but it’s not the kind of place you want to start practicing your breast stroke. A little kick and float every once and a while never hurt anyone, but it’s a place to relax. Needless to say diving, cannonballs, and dunking your head under the water is mostly frowned upon.
5. Don’t Splash
This goes without saying. You might see it if you are unlucky enough to go when a bunch of kids are there, but in general these are places of relaxation.
6. Don’t Worry About the Tattoo Rule
A lot of non-Japanese with tattoos may be put off by the tattoo rule, but it’s just something they put on a sign so that the management has an excuse to call the police if someone from the Yakuza gets out of hand.
That being said, I’ve been at public baths where a guy with a full-body tattoo was playing with his two year old son. Some of the other bathers seemed a bit nervous but the guy was a local yakuza member out with his family. No problem there.
I have also heard, however that some people are kept out because of body art, but usually a strategic bandaging can make things right unless the management is adamant about it in which case it’s probably not a nice, open-minded place anyway.
8. Don’t Forget to Wash
Everyone showers before getting into the baths, although you may spot some people dump a few buckets of water over their heads and then jump in. People may notice if you don’t shower before getting in and complain about it.
9. Don’t Bring Valuables If It Can be Avoided
Depending on the onsen or sento you visit, there may be lockers with keys provided free of charge, or there may just be wicker baskets on a shelf. Even if there are lockers with keys, they will often be the type used at fitness clubs that you can slip onto your wrist like a bracelet. Those keys could be lost or misplaced and fall into the wrong hands as easily as your wallet or camera could be picked out of a basket on a shelf.
If you have something valuable with you, you can discreetly ask the management to hold onto it, or just be very careful with the key to your locker. Japan is generally a safe country, but petty theft happens a lot more than people outside of the country realize.
10. Don’t Try to Find a Way to Peak into the Opposite Sex’s Bath
This is another one that should be obvious. Unless you have a private onsen in your hotel or ryokan room, baths will be divided by gender. The baths will also tend to switch each day or so so that each gender gets to enjoy both parts of a shop’s bathing area.
Every year people are arrested for trying to peek over trees or around walls or dividers between baths.
Another obvious part of this is not to take pictures of people or even SEEM to be taking pictures of people. When I check my cell phone in the locker room I always make sure it’s obvious I am NOT trying to film all the naked people walking around.
If anyone wants to ask or add to the list in the comments below please feel free!