I had this issue, there is an option in the settings for the Material Instance of the water where you can enable it to use fixed depth, I don’t believe it’s on by default. You will want to check both boxes.
This WILL cause the water to render under your terrain though, so if you’re worried about the performance impact you’ll probably want to go in and fine tune those splines on the terrain in the ocean water body object.
I wish I could remember which YT tutorial I learned this from to give credit but I don’t 
