I just got back from a 5 day cruise on the Celebrity Century. One would have been embarrassed to arrive in the Grand Dining Room in formalwear. I would have been one of them, but was glued to the television in the casino watching the Jets-Steelers game. In the casino there was literally no clue that it was a formal night.
On other lines formal or semi-formal nights are alive and well. Seabourn is no longer that formal as it has "formal optional' nights, but you will find that on most every night when a jacket is not required more than 50% will wear a sport jacket at a minimum.