There are other names for it like water biking but the basic concept is that you are doing a spin class in a pool.
According to people that run the class your can burn up to 800 calories in a one hour class. I suppose it is possible if it were a high intensity class.
I personally think it's a little gimmicky but the one real benefit I see to this type of exercise is that it would allow certain groups of people to bike that otherwise could not (like the elderly with balance issues or people with injuries that make it difficult to support themselves in that position.
I'd be interested to try it, but I'm guessing it will be pretty pricey. Has anyone tired Aqua Spin yet? If so how was it?