What really causes this concern? Did you grow up being told that father's don't bathe their kids? I did. There was a sexual undertone to the way my parents treated us. Daddy never gave baths or was in the room when we were in the tub. Daddy rarely held me once I started school. Modesty was offended the reason for the rules. After I became an adult, I saw the way other Dad's treated daughters and understood that my parents' concerns were over emphasized. I talked with my mother and learned that they were concerned about the possibility of my being molested.
Could your concern be related to fear that your husband could be sexually involved with his daughter?
It's common for step-mother's to feel they are in competition with step-daughter's for their husband's attention. Often, underlying the feeling of competition, are concerns about sexuality. Often the mother is unaware of those feelings.
I wonder why you don't participate in bathtime in a happy unconcerned way. You and your husband are combining two previously separate lives into a new way of living as a new family. Both you and your husband will gradually integrate your two different ways of behaving into a way that is comfortable for everyone. The way you worded your post makes this situation one of criticism and dissension instead of lovingly working out how you both can be confortable.
I suggest that counseling will help both of you learn a way of interacting in a more loving way. You will have the opportunity to be aware and let go of "old baggage" (previous experiences that affect the way that you feel now) that each of you bring to the marriage.
A person needs to be aware of the cause of their feelings in order to know how accurate to the current situation those feelings are. Your sense of something being wrong may be caused by old experiences and feelings or may be based on what is currently happening.
For example a mom may have been molested as a child making that mom more sensitive to anything that seems to indicate that possibility. I was only able to effectively investigate sex offenses, as a police detective, after I dealt with my own sexuality issues.
All of us are influenced, either negatively or positively, by our past experiences. I suggest you ask yourself why you are concerned that showering is more appropriate than a tub bath. Why are you not wanting the father to participate in his daughter's bath routine? What past experiences may be the reason for being uncomfortable in this situation?
Based on many years of experiences as an adult, I now feel comfortable about fathers and daughters experiences that frightened me as a child.
I've also been jealous of the time my husband spent with his son. My husband seemed more in tune with his son than with me. My feelings were unrelated to sex. They were related to my need to be a perfect mother. I saw my husband not setting boundaries that I thought were essential to his/ours son's well being. We divorced. His son became a loving and responsible adult without my involvement.
BTW, my adopted daughter still prefers bathes. She was 7 and I continued to help her with baths and hygiene until she wanted to do it all by herself. Same with my granddaughter. My grandson is 12. This week is the first time, at my house, he totally managed his shower from getting the right temperature to putting on his jammies. This change from total dependence to independence happened gradually. The final step before this, was my getting the right temperture and laying out his pjs.
My grandson is a high functioning autistic child with sensory processing issues. His developmental age is closer to 10 than 12 as a result. I suggest your step-daughter may have emotional needs for being helped with her bath. Same with her Dad. Divorce affects everyone in a painful way.
I also suggest that "normal" eight years old children develop at different rates. I suggest, based on your question, that it's OK for Daddy to help his daughter at bath time. I suggest that you gradually become involved in a kind and loving way.