Start your game.
Pause, then type grow old along with me.
There should be a 'ping' and the message cheat mode active will be in your bulletin log.
+ on the numeric keypad switches on invulnerability for your Walrus & Manta craft, and - deactivates it.
Repeatedly pressing or holding 7 on the keypad allows you to time skip, which significantly reduces the time taken to travel between islands. The enemy can still attack while you are 'time skipping' and you will be unable to defend yourself, so bear this in mind.
There are also some other minor features:
9 - Displays the current difficulty level
8 - Toggles co-ordinate bar on and off
6 - Displays the colour palette