When you are at surface (0m) you can use this view to see around your submarine :
If your submarine is not at surface, the screen will be black :
Move your finger on the screen to rotate the eyes left and right; you can also move the eyes up and down, to see incoming airplanes.
The zoom for deck view is fixed at 1x.
On the bottom of the screen there are other 2 buttons (Center, Follow) :
Center :
First select a unit in the map, then press this button to center the deck view to the selected unit, to see it at the center of the screen.
Follow :
Select a unit on the map and press this button to keep it centered on the deck screen; when “Follow” is active the button’s icon becomes green; press it again to release the “Follow”.