Scale over the large body of water in the "Pagalagos Island". Hover over the Skweek and press Option 1 to collect 3000 extra coins.
Scale on the small island directly to the right of the "Welcome Island" in the top right corner of the dark blue spot. Hover over the Skweek next to a palm tree and press Option 1 for two extra lives.
Select the normal game option. Move Skweek or Skruch over the "O" in the word "Gargouil Land" at the map screen. Scale the screen until your character can not fly lower. A snorkeling furball will appear. Carefully position your character over the furball and press Option 1. The Forbidden Islands may now be accessed.
At the Password screen, enter the following passwords to warp to the desired level:
There is a fourth secret character on the map out in the middle of nowhere. On the map, position yourself at the bottom of the R in "GARGOUIL' LAND" and zoom in all the way. Carefully move three pixels south, using the shoreline of "Vociferia Eatacus" as a landmark. Now go west until you encounter a pink whale. Hover over the whale and press Option 1 or Option 2. You will get the message "BRAVO CHEAT ON." The next time you go to the map screen, either by completing one of the islands or restarting a Normal Game, you will go straight to the ending.