HP: 16 (or 32, if it's too pointless)
It functions exactly like a corridor, except it has no entrance going up. Basically, the only reason it would exist is so that we could build awesome-looking decks on our ships.
Without a deck:
With a deck:
Maybe crewmembers could move faster on them, but take more damage?
I like that idea.
That's actually something that I've considered as well. Stupidly, the main reason it's not in yet is that it may interact badly with the boarding code that lets boarders walk on top of ships.
Functionality-wise, its major advantage would be that crew can move through deck modules even faster than they can through corridors. Its major disadvantage is the distinct lack of protection... :D
(Also, thank you for all the suggestions. Keep them coming!)