HARRY POTTER AND THE PRISONER OF AZKABAN (2004)
The visual effects animators were having trouble capturing the eerie, floating nature of the dementors, so to help them out director Alfonso Cuaron had a real dementor made. He put the dementor in a tank of water and filmed it. The way the dementor moved in the water was exactly the look Cuaron was going for. That way the animators had reference footage, so they could achieve the desired look. If you pay attention during the film, you can see that the dementors do, in fact, look like they're moving through water.