Again, just my opinion, but I think decent ventilation and wiping down excess water is all you should need to do. I mean, you aren't worrying too much about rain on the decks, right? It rains, they get wet, then they dry off again. Same on the inside. As long as you are not trapping moisture in the head I don't think you will have a problem.
I agree with Chris, it can get wet no problem as long as it dries out well afterward. I'd focus on lots of forced air movement in the space. A good sized fan on a timer that continues to run after the shower is over, and maybe a small solar-powered fan that ventilates the space all day.