Mama Safi
Air bed is a soft mattress that feels like you are sleeping on air while water bed is a mattress filled with water and so you sleep on water

Mimi
Water bed is not so tight on the surface, you can make waves on it. It's like having a sheet of plastic on top of a pool of water, you can imagine how it would move with the waves underneath etc. I only slept on one a couple of times in my life when I was a child, so can't really remember, but didn't really like the experience much - once I had to share the double bed with another little girl, and I was afraid to move as it made so many waves I didn't want to disturb her. Air beds I've used many times for camping - they're a bit stiff on the surface really, even if not blown up all the way, when you lay on them they become stiff. So neither water nor air beds are particularly comfortable from my point of view.

sensible_man
Water beds can be purchased according to how much movement you want. I had a 90% motionless and loved it. The problem is if you need to move them. Air beds are simply an air mattress on a frame. I have been using a Coleman air mattress in my waterbed frame for two years, and have had no problems.