Answer: Depends on location
Explanation:
I was curious about windows on deep ocean vessels so I looked it up. There are companies in Norway for one, that manufacture this glass, also windows/frames and doors on all sizes of craft. It is specialized glass with sound and fire proofing, seal proof, pressure tolerance w/relative thickness. Size and shape are customized up to 4 ft thick for deep sea vessels, like submarines /mini-subs, but usually 6-8-10-12 inches on cruise ships depending on location. The bigger the window dimensions, the thicker it needs to be, such as the viewing windows on the main deck. Steel walls are usually 18mm to 32mm to endure water pressure per square inch.