Forbidden Island - Extending to 5 players
I tried: you can, but each difficulty level is to be considered one lower.

In order to be able to have a chance to win with your friend if they have never played the game and that you want to start easy, you have to put the water cursor below the first line, adding a new level of difficulty: "extra-novice".

But if you intend to have more than 4 players regularly, may I suggest you buy another cooperative game that is suitable for 5?


Atlantis Rising (2-6 players) takes some of the same principle of drowning land, without being exacly the same game.
Battlestar Galactica (3-6 players) is a team vs team game, except you don't know in which team you are until the middle of the game.
Shadows over Camelot (3-7 players) allows each player to follow his own quest, but be sure to find the traitor and don't let him ruin your work!
Hanabi (2-5 players) asks every player to launch fireworks by playing cards in the right order (exactly like Ligretto) except you hold your cards backwards and everybody sees them except you.
There are 3319 co-operative games (when I write this answer) listed on Board Game Geek!