Enter these codes on the cheats screen:
TUSKFISH: Unlock Missions
BANNER: Invincibility
WRASSE: Bullet Shield
BOXFISH: Replay Items
PARROT: Invisible Soldiers
DOTTYBACK: Men With Hats
BETTA: Rubber Grenades
JAWFISH: Unlimited Ammo
BATFISH: Silver Bullet
CARDINAL: Achilles Head