An onsen has rules and they are not negotiable, because the water is shared. You wash thoroughly at the taps before you get in, you bring the small towel but never put it in the water, and you wear nothing at all.

The tattoo question is real and is changing slowly. Many bath houses still refuse tattooed guests outright; others allow cover patches, and an increasing number of hotel baths and private kashikiri baths do not care. Asking in advance — tattoo wa daijoubu desu ka — is normal and saves an awkward turn at the door.