Displacement does not work on infinite planes in Carrara, so it converts it to a regular plane. I think to get some nice ocean surface you should try using a terrain. Some of the filters are suited to that type of surface. To get a large expanse change the 18 mi size to 100 mi. in the terrain generator. Here's a sample test I never finished witha bad model. It's low res and rough , but you get the idea. I used caustics to get the light reflections.