Hi YoYo.
That looks like a known P3D texture problem that has existed for quite awhile. When certain land classes border or are close to water and feature black patches as part of their texture, the P3D rendering engine makes a mistake and renders those black/dark patches as water. Lockheed was informed on this long ago by ORBX. It seems to really only show up in a certain landclasses mostly used around Alaska which is a very small part of P3D's world. ORBX actually made an update to one of their Landclasses in SAK to minimize the issue which pretty much indicates that P3D may never get to that fix. There are a few other P3D texture problem in the Alaska area also that I wish Lockheed would fix.
Just gotta live with it....
Rod