Would it help from freezing if the water is moving?
If so ... I found this
http://www.birdsperch.com/alpewi.html
It might slow down the process some, but it won't prevent it. Good idea though. Of course, these birds here were having pool parties in the birdbath every night when the weather was warm. I had to fill it 2 or 3 times a day. I could just imagine how bad they'd be with a jacuzzi. LOL!