Acrylic is the safest choice, since it does not swell or delaminate. Sealed hardwood and bamboo hold up on a counter away from direct spray. Untreated plywood is the one to avoid: steam gets into the glue lines within weeks and the layers lift at the edges, especially near a shower.
How do you seal wood for a damp room?
What can you mount on tile without drilling?
What size suits a counter organizer?
Does an engraved bathroom sign need special treatment?