Enter these on the passcode screen:

TUSKFISH - Unlock All Missions
BANNER - Invincibility
WRASSE - Bullet Shield
JAWFISH - Unlimited Ammo
BATFISH - Silver Bullet
CARDINAL - Achilles Head
BOXFISH - All Replay Items
PARROT - Invisible Soldiers
DOTTYBACK - Men With Hats
BETTA - Rubber Grenades