Hi ghost
if you want find for Japan your shores like before,the default, you must delete or move in a save folder ,for later when a new Japan scenery will be made with new coastlines fitting with the mesh,the two watermask bgl CST4a10b_m.bgl and CST4a10c_m.bgl
for the CST4a10a_m.bgl I don't know if it's best to keep it even if a very small nw part of japan is inside ,this bgl cover the entire Korea with no shoreline if i remember, i ve not flight over for a while????,it's in the no shorelines area that the rhumba water mask show today their interest ,if you may keep the default mask where there is shorelines in waiting for some new sceneries mostly adapted with the R's masks and mesh you could check each CSTxxxx bgl to know their location and choice if you leave or not them with Tmf viewver ,you can find it with an old fs? sdk, there is one in the g2k folder and .....I don't know where to download it directly ,avsim no ...,SOH no ....,so here the Tmf "bgl " viewer tool.....in attachment
change the extension .pdf to .exe
with it you can check each bgl in the R's watermask scenery folder and you will see the coresponding area then you can choice to keep them or not...or organize them in folders as euro africa asia pac etc...
PS: i've just flight over Korea, CST4a10a_m.bgl,there is default Beachs ......
JP