Everything. Someone could bring a lawsuit up years later, and logs would be necessary to determine if they had standing.

The aforementioned optical media storage was specific to the nuclear reactor and electric plant; I think everyone else’s were stored differently. Not positive.

EDIT: sibling comment below mentions performance data. Yes, that too. I graphed (nuclear) fuel consumption on one underway, and was surprised to find it didn’t match expected. My Captain was also surprised, and thrilled, because it meant he got to be more important (fair enough; who doesn’t want to be listened to by their boss?)