The biblical text uses the word window twice in the account of the ark, each time using different words. This suggests to me that Noah built two different styles of windows in the ark.
First, the instructions God gave Noah specify a window as an overhead light source:
A light/ window/ opening/ noon [tsôhar] you shall make for the ark, and to a cubit you shall finish it upward/ above; and the door of the ark shalt thou set in the side of it; with lower, second, and third stories shalt thou make it. Genesis 6:16
The second word is identified as a window that Noah designed.
And it came to pass at the end of forty days, that Noah opened the window [challôn] of the ark which he had made/ appointed. Genesis 8:6
The text uses two different words because there are two different windows.
The first window is an overhead source of light, the word is the common term for “noon,” the point at which the sun is directly overhead. Notice also that the location of this skylight is said to be “above,” in contrast to the door “in the side” of the ark. This seems to point to an opening with some sort of transparent covering so that light would be admitted.
The first word is the Hebrew tsôhar צֹהַר, which occurs 24 times in the Old Testament. In every other occurrence it has the sense of midday. For that reason, we should allow it to mean the same thing in the account of the Ark.
The word in chapter eight is challôn חַלּוֹן, which occurs 31 times in the Old Testament. This word always seems to refer to an opening in the side of a structure out of which one could observe events outside.
This is the word used of the window out of which the king of the Philistines observes Isaac “sporting” with Rebekah his wife (Gen. 26:8). In the case of Rahab and the two spies it is a window in the side of the wall (Josh. 2:15). Other references repeat this sense of something in the sidewalls of a structure out which observations could be made (Jdg. 5:28; 1 Sam. 19:12; 2 Sam. 6:16; 2 Kings 9:30). Time after time challôn is an opening for observation and for admitting air for ventilation.
So what might this window have been like in the ark Noah built? It may have been a small water-tight porthole that Noah needed to open to release the birds. But in my opinion, more likely it was a ventilation opening that was under the eaves of the ark, but possibly protected from running awash by scuppers.