What glue would you use to glue wood (polyurethane finished) to pvc shower walls for shelving?
Wood and water are a poor mix. There is a really good reason that you do not see show enclosures made out of wood. It is also the reason that you will not find much in the way of wood shelves for use in showers.
When wood is used in showers and steam rooms for benches and shelves the wood of choice is teak.
If you are bound and determined to put maple wood shelves in your shower then make sure that when you apply the polyurethane that you get it applied to all sides, edges, corners, tops and bottoms before you mount the unit.
You will have to experiment but there is a good likelihood that a silicone type adhesive may adhere well to the smooth polyurethane surface and to the smooth shower wall. Before committing it would be well to make a small test wood piece that you finish with polyurethane and then see of the chosen adhesive will stick to it. There are also a family of construction adhesives that will adhere to some types of plastics and finished wood surfaces but again try before committing.