Basically defog every area and small island on the map
Seems everyone has a different experience with this, I had been to every island no matter how small and still nothing popped, so decided to just de-fog the rest of the sea and after the last bit the achievement popped, so I'd recommend defogging the whole map before worrying that you're bugged.
Multiple people have had the same experience so I'd say at this point its required to defog the whole ocean and not just land areas.
I'll list a few spots that are easily missed (at least by me)
Kithairon Foothills in Megaris
Mount Koryphe in Argolis (there's two parts to this area)
Kradia Isle under Samos
Chytra Atoll under Kythera Island
Valley of the Two Kings in Lakonia (Credit to Mr 4T7)
Marble Bay in Naxos, people saying it may look like you've been there but might not have so double check! (Credit to ManlyTorchic)
Typhons Revenge in Melos, both islands need to be touched (Credit to OwnedByOmar)
Any others please let me know and I'll add!
There are claims saying that the Volcanic Island Nisyros has two defogging areas so run around that to make sure its all clear, those islands are a pain as their colour is pretty similar to the fog colour.