Noach's boat is called a teva, which as far as I know means "box". (The same word is used in later Hebrew for coffins and closets (as in "hayored lifne hateva").)
It's described, at least according to Rashi's understanding, as having three stories and a roof that was sloped to come to a cubit-wide (seemingly flat) roof at the top.
I have no reason to think the top story of the boat was any narrower than the second (which would form a deck), although presumably eight people (who lived in the top story) wouldn't need the whole area. Arguing against a deck is that Rashi's reason for not having a flat roof (that water not pool) would apply equally to a flat deck.
Yet depictions of the boat typically show it with a deck. Is there any basis for saying it had one? Is there any explicit source to say it did not have one?