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