Ok. First of all, I haven't found the answer, but I do want to show you how a problem should be troubleshot and what kind of file you should provide others when requesting help. First, I removed all the scene objects not needed to show the problem, which left me with this:
I also deleted the polygons and points in the Beach plane object that aren't seen by the camera. Then I deleted all the unused\ materials that aren't needed to render the scene which left me with this:
Then I removed all the image files in the water material, as they aren't needed either. And that left me with this render result.
The red arrows indicated the black line artifact. I then saved the file as a new file which is about 1/50th the size of your original file and uses no image files, but still shows the artifact. I also turned off Global Illumination, as that too isn't needed to see the artifact. .
And here attached below is the resulting file in case anyone wants to troubleshoot it from here.
landscape quixel ross masonMV4.c4d