Doesn't a joint (roughness) change suddenly
and isn't Flanker's sea surface texture visible
near the seashore, by the default?
It is the phenomenon which happens since the difference in the texture size of a long distance
and near is large.
On this page, the method of the sea surface texture which I devised is introduced.

Example, If You play high detail setting.
First, backup files
Flanker folder\Bazar\Terrain\Surface\High\Textures
and open sea-128.bmp in this folder.
(use paint software etc,)
next, open sea-256.bmp
Then, copy sea-128.bmp and paste sea-256.bmp.
It makes like a left picture.
And overwrite preservation.
The fine sea was made now.

the folder same next -- 0_5.bmp 0_6.bmp -- it finds that there are some texture files near the seashore of 128x128 size named ...
From now on, those files will be processed and it goes.

0_5.bmp should explain as an example.
A paint tool is used.
First, it expands to the size of 256x256.
Next, by free hand selection of a paint tool etc., range specification of the portion of land is carried out, and it is copied.

And the Sea-128.bmp file made before is opened, a position is united and the copied land portion is stuck.
If it can do, overwrite preservation will be carried out by "naming and saving a name" at 0_5.bmp.

The above-mentioned procedure is processed into the texture with the coastline.
If you have interest, will be try although it is the work which requires perseverance very much.
In addition, even if it changes the file of 128x128 in size into 256x256, Flanker reads as it is.
(It is in the case in the graphic card of GForce256 and MX) Since Flanker's texture is BMP file, it is processible so that he may consider.
It is easy, You can modify to try many things in your idea.