When you know you are on a small island, you can take very simple measures to eliminate early military needs from your building plans; just place a few scouts/warriors on hills to clear FoW, and you don;t have to build any more military except for happiness. This then allows you to focus on extreme builds, such as slingshots that can propel you ahead of the AI and ride that to victory without much thought.
Naval combat is not particulary difficult to understand; keep boats on coast for defensive boni, except for the resources on theocean that you have to protect. Keep boats on "choke points" so you can see the AI coming to attack you then sink them out in the ocean (or not if you can't be bothered, because the AI is so inept at attacking via boats.)
The Ai launched invasions in my game fairly well, it's only problem was a centralized defensive network, and that I had better units due to me going balls out and teching into the future faster than the AI could do it.
Knowledge of the map type gives the human a bigger advantage than the AI, because the human knows what the map will roughly look like, whereas teh AI has no clue and still has to play it safe hence slowing itself down in the early game.