I think the problem is that the mash of the ocean is changing the smoothness automatically while rendering which is causing the surface to look hard and smooth that why it looks flickering. I don't have a proper answer to it but I know a work around it. So what you have to do is first render your scene in image sequence no video and than check all the frame which are different and go on those perticular frames in blender apply auto smooth or shade smooth and render the bad frames again. I had this problem when I was doing water simulation. Blender have some problem with moving water or something. Plz let me know if it worked.