That is a gas mask, and the game controller. You can’t see anything at all, and you can only hear the creepy sounds that come from the game itself (everything else is canceled out by the headphones). In Deep Sea, you are underwater and surrounded by unthinkable creatures coming at you from all sides. An AI character tells you which way the monsters are coming, more or less, and you have to try to shoot them down with your joystick. If there’s a hit, you’ll hear a scream — if you miss, all you will hear is your shot drifting away into the sea (and, eventually, your own scream).
The game is extremely claustrophobic, and this is all quite intentional: In this interview, the game’s creator admits that “It’s meant to be a difficult, frustrating, scary experience. I saw somebody rip off the mask and I realized … I think I’m on to something here.”
In fact, the more scared you are, the harder the game gets — the game actually keeps track of your breathing and plays back an exaggerated breathing sound that increases the more nervous you get, sooner or later drowning out the game’s commands and getting you killed. Even if you’re not claustrophobic at all, you might be after playing it.