Public baths and onsen are for soaking, not washing. Use the seated showers first, scrub thoroughly, then enter the tub clean.
No swimwear. Most onsen are gender-separated and bathers are nude. A small towel covers you on the way in; place it on your head or beside the tub, never in the water.
Tattoos are still restricted at many traditional onsen. Smaller cover-up patches exist; some 'tattoo-friendly' onsen welcome inked visitors — search ahead.