Put a speaker directly over the shower area if you intend to listen while in the shower.
This is something I have learned from experience.
You do want to seal around the speaker to ensure moisture doesn't get into the attic space, but you will have to crank speakers up to hear them in the shower if you don't do things this way.
I used OSD outdoor rated speakers for wet locations. I did have it fail after about 5 years on an installation, but was able to replace it with a similar model.
Otherwise, you can pretty much run 'mono' to several ceiling speakers in a bathroom and be pretty happy with the sound. You just will be unhappy if there isn't a speaker right over the shower. The shower doors simply block to much audio.
That said, you don't care about cranking it up to hear it, then by all means, do so.