I think the best thing to do is to be honest and say where you were before. I was in a very similar situation. I had been at my first hospital for about two months when I realized things really, seriously weren't working out. It pretty much just wasn't what I was looking for. I was completely honest about everything, my reasons for leaving, and I gave references at my other job to call. I left on pretty good terms with my nurse manager. My new job really appreciated that I was honest about it, and in my interview I made the best of it all, telling them about my clinical experiences at the first job. I'm pretty darn happy at my new hosital now.

Your six months did count for something! So depending on how things were when you left your first job on the second, why not give both units as a reference? I mean if you did your job well, how could anyone say anything bad about you? Anyways, I would at least list the first unit as a reference. I think that would be perfectly valid if they could give a good picture of your work ethic.