I am presuming the shower walls are not suppose to get wet so other than the mods posted here I am wondering if this spray product might work.
http://www.homedepot.com/p/Rust-Oleu...8146/205160314
There is another 2 part application from Rustoleum but it dries milky white, so I do not think that would be a suitable option. I am not looking for it to replace the shower curtain, only to be a protective barrier for any water that might get on the walls otherwise.