Good thread, I have always said that "true difficulty" is a mixture of many game factors, which include difficulty level, play speed, leader traits, map, etc.
Of course, individual play preferences also play a role. Speaking for myself, I do better on watery maps where I can build the GLH and REX to reasonable islands and lots of coastal sites on the main land mass. I do less well on Pangaea type maps with lots of AIs right in my face.
Eh. I tried playing Watery Non-Isolated maps, didn't do to well. But make it Isolated, and suddenly, I may not be the power leader, but I out-tech everybody and have the infrastructure and tech to out-military Shaka or Monty in a matter of turns. (IE: Rifles and Cannons vs. Muskets/Maces and Cats)
So IMO: Watery Non-Isolated - +.5, Watery Isolated - -.5

), making that the first wave of attackers will probably never be reinforced in that war. If you can hold the first attack, you won the war..... besides the fact that the AI will rarely re-embark a SoD for attacking other landmass.