I don't think believing the main witness is telling the truth is necessarily the "simplest explanation". For example if I walk up to a man holding a gun and see a bleeding man on the ground next to him, and he says that a circus truck drove by and a monkey jumped off it, ran up, grabbed the gun, shot the man, and then the monkey tossed the gun to him as the monkey grabbed a rope dangling off a helicopter that took it away, which is why he is standing there holding the gun. The simplest explanation would be the man shot the bleeding man himself.
"I thought there was glass" is SA's monkey story IMO.
I am not addressing the 'did not care' vs 'did not know' the window was open because I think he did care, he wanted an open window. I think the simplest explanation is he went to the window
because it was open in order to look out with the clearest view and perhaps to get some breeze.
Looking at some of the pictures at night of the windows or even daytime still pictures from certain angles and distances, without imagining actually being there, makes "I thought there was glass" plausible on the surface. But the still pictures remove sound, smell, sense of breeze, and the spatial nature of 3D vision while moving around. I have not been on a huge ship like that but I have been on boats, docks, peers, beaches, and in a sunny afternoon the sense of being 'outside' vs inside is very obvious and strong. That sense of the outside would exist next to a huge open window like that. The video of the YouTuber walking the ship best demonstrates this as he holds the video camera to the open window. There is even an acrophobia feeling just from the video when at the window. Imagine the video camera is your head and you are actually there how scary that would be to look down. How would it be possible not to notice that.
That is why I hope if it does go to trial the jury visits the ship as part of the trial. Or hold the whole trial on the ship like this (maybe not practical for a civil trial):
I think it would be a lot harder for SA to testify that he thought there was glass while on the ship in the afternoon with everyone able to walk up and look out the actual window.