Can you use this on a glass shower door? Does a high humidity environment affect how well it clings to the glass?

Yes, both adhesive and static cling films can be installed in humid and moist conditions. If being installed on a shower door, mounting on the outside of the shower door is recommended. High humidity does not affect the durability of the material. If concerned about water seeping around the edges, you can apply a clear silicone caulking to the edge to prevent this.