How common are wet rooms in the US?
I worked as a contractor for years and there is really only one rule... There is always a buyer, if you wait long enough.

Having said that, 'wet rooms' are not very common in the US at all.  In the 100's of homes I've worked in I've only ever seen 3 of them.  My advice here is two fold. 

First: If it is worth it to you do it and don't think re-sale, you may lose money, but if it makes your life a lot better for the years that you live there it's probably worth it.

Second: If you decide to go this way, do it right! Make sure you take lots of photos to show people, have (and keep) all of the permits, and make it look perfect.  People who would consider buying it even if they may not be familiar with a 'wet room' will run if it looks shoddy or sub-par.