Actor get flickering if they are too close

I am now creating an extremely large open world using landscape.
And I create my ocean using a large plane actor and its Z-location is very closed to the bottom part of the landscape.
The two actors is approximately only had 50-100cm difference between them.

As a result, their is flickering problems when the camera is over certain height, shown as below video.

May anyone tell me why this would happened and how can I solve this problem?

Thank you very much.