I'm not quite sure why you are asking such a question. If there is any significant difference betqween informal and smart casual, it certainly escapes me, and I'm quite sure no-one on the ship would find any difference.
On Fred Olsen ships there is a significant difference between 'Informal' and 'Casual'. On Informal nights gentleman are requested to wear a lounge suit, or jacket and tie. On Casual nights gentleman can dispense with a jacket.
Agree with Mr./Ms Stone. There are normally two formal nights each week. Casual nights tend to be the first, last and when the ship is in port overnighjt.
Just returned from a fantastic cruise around the Adriatic...... informal evenings just mean a tie or jacket so you could get away with no jacket. I took one jacket and several ties and this was fine. Enjoyed the three formal evenings when everyone was extremely smart and most people seemed to try hard with outfits for the themed evenings. All the best Gavin Turner.