Captain's Key

The Captain's Key is an item that can be located in a small room in The V-100.

How to obtain
At first glance, it is seemingly inside a petit room with no real peril. But when the room is approached, a red alert will sound, and the door leading to it will slam shut. Then you must steer the submarine away from obstacles - or else you get fluffed. After that is cleared up, you can go retrieve the item.

How to use
After obtaining the First Mate's Key as well, it can be used to unlock the safe containing the Torpedo Launch Codes