Asphyx

Asphyx is a short side-scrolling game in which the nameless pixelated protagonist needs to navigate a maze of tunnels and underwater passages. There are no enemies and the protagonist cannot die. However, to make things interesting the author suggests that every time the protagonist goes underwater, you, the player, should hold your breath. When you are out of breath hit the Esc key and the protagonist will drown. However, the game doesn't punish the player for not doing so and just keeps playing.