First Mate's Key

The First Mate's Key is obtainable in The V-100.

How to obtain (in all other versions)
You can find it after going through the torpedo that a beaver was apparently imprisoned inside. It is laying on the floor.

How to obtain (in Viggo's Revenge)
It is lying in a small room connected to the Sonar Steering room. Approaching it will force you to initiate the minigame. After clearing the minigame, you can retrieve it from the room.

How to use
It is required to unlock the safe containing the Torpedo Launch Codes, along with the Captain's Key.