Addressing the original concern.
Generally, you just add a 4vert sheet thats 8km long and runs a very simple material which simulates the ocean at a distance.
When a single sheet is too heavy, you create an Ngon with N tris. Take a single tris, and distribute instanced copies aorund in a circle without gaps.
Solid terrain(s) are then generally continued for about 8km in low detail, so that when a player reaches the edge the world doesn’t just end.
8km is rather arbitrary.
It is drived off the fact we usually only see 5km in a percect plane to the horizon, and that possibly being on a tall building or flying may cause more distance to be needed. It reallly depends on the overall height of the map plus the flying altitude.
For inatance, if your map was from sea level to K2, you’d have to have much more than 8km past the boundry to prevent the world from looking like it just ends when overlooking from the highest point.
Now if your map requires it, you can also cut the number of Ngons that represent the distant ocean down so they only appear in the direction they should…