I think it's kind of neat, actually. I could see it on either gender. I think it would be usable, but there would be people who would think it was dumb/funny. I couldn't see it getting more popular, it's just a short little syllable name.
I actually thought of bay leaves first, then the horse color, then the body of water. So with all of those swirling around in there, I don't know what to think! I'd imagine a girl named Bay would be smart and quirky and outdoorsy.