Do you mean inside the shower/bathtub?

I am looking for inside the shower/bathtub area.

The only thing that comes to mind to use inside the shower/bath area would be tile with an attractive edging or border at the top. You could go simple or get creative by creating patterns with the tile. It could be any height you like. You will have a problem if any gypsum walls are likely to come in contact with water. As you know, wainscoting only goes up to a certain height. In that case I would go up to the ceiling.