As soon as your number of subs reaches "0," pause the game and do the Level Skip code. This starts you with 99 subs. You can do this once per game.
Double Scoring
Pause the game and press A, B, B, A, Right, Down, Up, Left, Left.
Full Systems
Pause the game and press A, B, Select, A, B, Select, A, B, Select, A, B, Select, A, B, Select, A, B, B and A. You can do this once per game.
Invincibility
Pause game play then press Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, A, A, A, B, B, B, Select, Select, Select, Right, Right, Right, Up, Up, Up, Down, Down, Down, Left, Left, Left, A, B, B, A and the bottom of screen will turn green. Note: this code may only be enabled once per game.
Level Skip
Pause a game, and press A, B, Select, Right, Left, Left, Right, Select, B, A, B, B and A. You can do this once per game.
No Forced Scrolling
Pause the game and press Up, Up, Down, Down, Left, Left, Right and Right. This allows you to turn the scrolling movement on and off.
No Inertia
Pause the game and press Up, Down, Left, Right, Select. Repeat the code to turn the inertia on and off. With inertia on, the sub coasts a little bit before stopping. With it off, the sub can stop on a dime.
Slow Motion
Pause the game and press Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, A, B, B and A. Perform the code to toggle between normal and slow speeds.
Toggle music
Pause the game and press Select to disable the background music. In REV0 versions, you'll see the bottom of the screen change to green to confirm this, but note that this cannot be reversed. In REV1 and European versions, you won't see the bottom of the screen change, but you can toggle the music both on and off.