That could account for the belt FX in Beyond the Farthest Star,
but many of the belt FX in The Ambergris Element
look like they were done using a different technique. Maybe they needed a different technique in that episode (possibly airbrushed cells placed behind the character cels?), in order to have the "rippling" water wave distortion effects apply to all the elements in a frame.
By the way, how did they do the "rippling" water wave distortion effects? Was a refractive medium placed between the cels and the camera?