A bathing suit is fine for breakfast and lunch-as long as it isnt wet! Especially in the Windjammer when people will be heading off to the beach, tour, or pool. For a man-wear a shirt and shoes, a woman-wear a coverup and shoes.
We were on Serenade of the Seas this past April & bathing suits & such were not allowed in the Windjammer. You had to be covered up & wear shoes, sneakers, flip-flops, etc. There was even a sign posted & one woman was turned away for lunch when she tried to enter with shorts & her bikini top.
Just make sure you have a t-shirt/shorts on & you will have no problems. I know they did not allow bathing suits in the dining room either. They were very strict on that.
So as long as you are wearing shorts and t-shirt or tank top, you can have your swimsuit on underneath? For guys, most of the swimsuits look just like shorts these days, are these okay with a t-shirt or tank top? We never ate in the Windjammer on our Monarch cruise last year, we were in Ensenada the first day and had burgers on the pool deck the sea day, but I wondered how they handled the "no swimsuit" rule in the Windjammer, with so many people being out at the pool. Of course, I would expect to wear something a little nicer in the dining room at lunch, nicer shorts, no swimsuit underneath, etc.