Had a wooden shelf in our shower.  It was made out of some sort of exotic wood and did last a decade or more.  Decided to replace it when it started to fall apart.  After much searching for proper wood for this sort of environment (is there any such thing?) I decided to go with a cedar 2×4 I had in my shed and some fiberglass resin.  Used this stuff once when I was in high school to make a darkroom trough and it is pretty tough (and looks good too).  Will send an update when I get it bolted to the shower wall.