We have just come back!!! its brilliant did Canaries back on the 31st May, now your answer in the main dinning room ok for shorts and tee shirts but not vest tops, buffet restaurant is ok for shorts/tee shirts, have a great time, you will!
Breakfast in the dining room is served by waiters - far more civilised than the Windjammer buffet. Lunch is only served in the dining room on sea days. They have something called 'Bistro 30' which is a menu of wide variety of food and cooked to order - and the idea is you are out in 30 mins. Food in the dining room is far superiior to what is served in the buffet IMHO. Remember you can also go to the Johnny Rockets american diner for lunch - $3.95 cover charge but well worth it. Great burgers and endless onion rings.