We packed a sport coat, tie, nice shirt, and Dockers for our DS 16, and when we got on board, discovered DS had outgrown the coat. He looked just fine in a decent shirt, tie and Dockers.
However, it should be noted that tuxes were rare, about half the men wore suits or sport coats and ties, and everyone else, normal clothes. It isn't enforced.
One couple kept us entertained the entire trip. They were two tables away from us. They wore sweatsuits to dinner on formal night!! I gather from the grapevine they were a bit difficult for their wait staff. They didn't get out very often, so anything more complicated than steak, chicken, fish, spaghetti or hamburger on the menu required a detailed explanation from the wait staff, and they usually sent their first choice back because of some problem.....one night the steak came covered in sauce, and the guy wanted to know why anyone would put sauce on a steak!!!