Undersea Magic is the "normal" animation that's on the screens on the Fantasy. Not particularly a "show" in the typical sense. The screens show underwater scenes, that various underwater characters randomly appear in.
Animation Magic is more of a show. At the beginning, after you've been seated, you will complete a drawing to be included in the show, and your server collects them. At the end of the meal all the drawings are compiled in an animation show that's on the screens.
Each will be once on a 7 night cruise.