If something is in your water it WILL get into your skin - and even more so when your skin has been in warm water to make it even more permeable. It is estimated that 91% of our exposure to toxins in water comes through the skin. Besides any steam inhalation, it is the contact with the skin that exposes us to not only fluoride but chlorine and all the various hundreds of toxins typically found in municipal water supplies.
Think about it - a great many medications are applied trans-dermally because the skin is is such a great place for apsorption. That is why we are warned to immediately wash and rinse our hands or skin whenever we come in contact with anything dangerous, such as a pesticide. If I owned my house and lived where the water supply was fluoridated I would seriously consider a filter such as the one I found here:
An option for those who don't own their house or can't or won't spring for the cost of a whole house filter system, might be a kitchen filter and purchasing a good portable shower system with heater where you could use water that had been filtered through the kitchen filter (or your bottled spring water) and place the unit in - or even better, outside - your bath to use in place of showering from the tap.
Here are a couple examples of camping showers that could be used - provided you were able to ventilate the bath of the gas fumes, which otherwise could be a problem. Ideally one would be able to locate the main shower unit outside house and run the shower head and hose into the bath, but that might not be feasible.
To avoid gas fumes and any other concerns about having a propane heater going in the bath, I imagine that someone who was a bit of a handyman could also rig up a system using s kitchen type filter or a reservoir of filtered water and adapt it to a tankless heater such as this one: