I suppose the game would end when one player controls 90% of every ocean... Just continuous, fully supplied, stationary armies waiting to be deployed, and countless city festivals to take the remaining 10%. If a conquer attempt happens, it would draw the attention of this allmighty godlike player, and defenses from the twenty nearest cities would come to the rescue, and another 20 would counter-attack.
For the most part, I think the player would be most focused on keeping his festivals up and running...