Don't put your hair in the water at the onsen.
Visitors must wash themselves off before entering the onsen,
with plenty of soap and with use of the hand towels provided.
Most onsen provide stools to sit on when washing yourself.
Be sure to rinse the stool off afterwards
Coin lockers are used to put belongings in; do not bring them into the onsen with you
Swimsuits and other clothing are not allowed in the onsen.
No photography is allowed in the onsen.
Don't let the hand towel in the water;
if it falls in, wring it out outside of the onsen.
Water is for soaking and relaxing, not splashing and swimming.