Ninja Gaiden Cheats

On level 16, get to the boss, and lose to him. Now, continue, and get to the Dark Dragon Blade. During the cutscene when you get it, press start, and the cutscene where you go to the final boss will play.

Approach a large body of water and just as you hit it tap A to run on the water. Keep running until you hit a wall and start sinking slowly. Eventually you will hit the bottom and if you keep running you will be able to breathe underwater.