Start game as usual. During gameplay press F9 to go to console,
then enter the following cheats followed by ENTER:

DESTROY ALL - Complete Level
NEXT LEVEL - Instant Level Skip
GOD - Toggle God Mode
SET DAMAGE 0 - Restore Health
SET BULLET - Get NUMBER Bullets
SET MISSILE - Get NUMBER Missiles
SET DC - Get NUMBER Depth Charges
LOAD LEVEL - Load Level 1-99
RELOAD LEVEL - Reload Current Level