Nami used her body just to have Momo agree with her. So yes there is something very wrong and sus about it. The fact Nami knows this makes her showers even weirder. And yes regardless of whether its a man or a woman, an Adult (especially a stranger) showering with an 8 year old is WRONG !!!!
How would you feel if it was an Adult Man showering with an 8 year old girl ?
I'm sure there are million of people (men or women) who are showering with their kids out of necessity without being any problems mate. I think you are really over reacting in order to take a shot at One Piece.
I'm usually the one here explaining why some things are wrong or toxic, and indeed, Oda's portrayal of Bonney is really borderline. But here, you guys are inventing literal stuff out of nowhere.
You could talk for example how Oda is making Sanji peek in women's bath or the fact that Oda is depicting some transgender as gross pervy boy chasing individuals, but you prefer to overreact on Momo having a bath with Robin or Nami...
Come on guys.. you can do better.