All the series with mixed bathing scenes/ episodes, hopefully more than just short and accidental.
Pic is from Ikkitousen ep 1, Hakufu isn't embarassed. Later on they visit a hot spring, in chapter 9.
I believe Pico to Chico has mixed bathing.
This is my fetish. I thought I was alone.
Most people seem to prefer when girls are alone. Fuck that. If I just wanted to see naked girls I'd go on Danbooru.
Rosario+ Vampire Episode 7 has a bit of it,
The most common version I'd guess is the "i'll wash your back" surprise entrance by the girl, but I wish there was more crowded and relaxing nakedness in springs or a large bath.