I agree that this is an issue with overlapping geometry, but I wouldn’t merge the geometry to fix it. Merging it would make sense if you made this yourself and didn’t intend for anything to overlap, but this was probably an intentional choice made by the modeler (maybe it was only meant to be viewed from the top).
Anyway, my guess is that the land and water are meant to be separate geometry that is supposed to clip through each other in certain spots. The issue comes from geometry along the sides occupying close to the exact same spot. The render engine doesn’t know which of those two to render in front, so you get that flickering. It is called z-fighting. I would just move the over lapping geometry slightly, so that they don’t occupy the same space.
I am going to assume that the geometry is all inside one object. and that you are using the default keymap. Select the object and go into edit mode, select one vertex along the side where the issue occurs, then press ctrl + L. This command selects all the geometry that is connected to that vertex, so it should select the entire portion of geometry that is overlapping with what it’s z-fighting with. Now press S and type 1.001 then press enter. This should slightly scale that geometry away from the other geometry and keep them from being too close together. If the geometry is in multiple objects, then I’d select one of them in object mode and do the same.