I thought of shaders - as far as I know, they're as issued, but they equally may very well not be. I'll take a look. In any case, I'm nowhere near Turkey - in MAW, I had this off the Libyan coast and around Malta. In RS, Hawaii.
The Z bias sliders are set to the values used in the "enabling high resolution textures" tutorial from GC.
<ZBiasBitsResolution val="22"/>
<ZBiasTerrainDecal val="0"/>
<ZBiasWaterPolyNear val="1"/>
<ZBiasWaterPolyFar val="0"/>
<ZBiasWaterLineNear val="1"/>
<ZBiasWaterLineFar val="0"/>
<ZBiasFlatEffects val="25"/>
<ZBiasShadow val="0"/>
<ZBiasTerrainAlphaNear val="0"/>
<ZBiasTerrainAlphaFar val="0"/>
<ZBiasObject val="0"/>
<ZBiasObjectFar val="0"/>
<ZBiasEffects val="1"/>
<ZBiasClouds val="1"/>
Later:
I changed the ZBiasBitsResolution val to 15 (as set automatically by the config tool) and that solved it in MAW. In RS, I continued to have the same problem despite changing the Z bias resolution until I copied the shaders from MAW over, but rather stupidly, I let it overwrite, so we don't have the guilty file!
Interesting that both mods should cause the same problem.