The level headed responses in here are refreshing.
The demystification of the body, I think, is a positive endeavor for parents. It is necessary, in my opinion, to fight this sexual shame mentality the recent generations have been forced fed. Within some basic limits, however you feel is appropriate for you and your family is exactly the guidelines you should follow.
I do not get in the tub with my 3 year old daughter (and won't my son when he gets tub age) for technical reasons (I'm 6'1 and our tub is small), personal reasons (I don't like baths generally) and because I don't feel it's a necessary part of our demystification process. I bathe her though. I sit on the outside of the tub and play with toys, splash, etc.
My wife will occasionally take her in the shower. Sometimes, we are all in there together after swimming in the pool. These are things we are comfortable with.
I don't have the all the answers or even a few answers. I know that we were met with an interesting development 2 days ago when I didn't realize our daughter had come into our bathroom, stood directly in front of me, while I was shaving in the mirror and poked me, with her index finger, right in the member! I never even saw her come in! Imagine my surprise. My reaction was a violent protective arm sweep that damn near clobbered her! When I realized what had happen, she was laughing, my wife was rolling on the floor, the dog was barking, and my face was bleeding from were I had dragged the razor across in my attempt to ward off, what I thought were penis stealing gnomes or something!
On topic to close: do I personally think it fits my current level of comfort to be IN the tub with my daughter? No. But if it does you, that is all that matters.