This is displayed correctly (ss 2). This is the border of high quality ocean and far mesh. You can adjust it (size), but this is how it looks on the border.
You probably have a bad World Partition setup, or you have the entire world loaded after opening.
Set the graphics to low in the editor and load the selected fragments. Loading the entire world can take ages if there are a lot of actors, but the ocean and landscape alone do not cause delays in 99% of cases - I have an island on the ocean almost 4x4 km with thousands of actors and it loads without a problem. I also have 2 large lakes. My hardware is weaker than yours.
Foreign plugins - this is my personal opinion - I would give up. I also had many problems (different) with the ocean, but overall it works fine, the problems are the lack of documentation or my misunderstanding.
Try it on an empty Open World level (the one with landscape) and see if you have the same problems.