I don't think naked bodies are dirty. Inappropriate, certainly, depending on the situation.
I come from a very private family. Exceedingly, extremely private. Therefore, if I found out my dad had showered with my son I would be 100% sure that my father had lost his mind. If my mother showered with a hypothetical daughter, I would feel the same way. It's just something my family would NEVER do. *I* never showered or bathed with my parents, if they did it with my kids, I would think something was up. Not molestation, but some sort of temporary insanity!
My ILs, while they break every other boundary in the book, would never shower or bathe *with* their grandkids. Just not their style. That's why this would raise real red flags for me.
Also, in that scenario, I can't imagine ANYONE being comfortable-- neither my son nor the grandparent he had bathed with. So it would be awkward and uncomfortable for everyone involved. If your family is cool with nakedness, then that's a different story, obviously.