[ACN]

From what I have heard is that you can hide/cover it with your towel, and get inside, (not legal).

The tattoo deny is not because of illness from what I have heard,
but since most Yakuza have tattoo they say no tattoo to onsens, since then the Yakuza can not enter.
like other says, it is not anti-tattoo, but anti-Yakuza.

So waterproof cover will not work here. you will have to hide it from there vision.
(if im wrong please correct me)