The observation deck is a room with a large glass floor through which the player and any surviving companions witness the aliens firing their death ray towards Earth. Initially, it is accessed from the space walk. Later, the player can teleport to/from the observation deck and the engineering core.
Below the observation deck's glass floor, in the center of the room, which is facing Earth; you can see what appears to be the East Coast and Central America extending down below Florida and upwards north of Canada. Including this, you can make out the Great Lakes and Michigan.
This is one of many areas of the ship that will not be accessible after the player has completed the final quest.