I call BS on this. I am Sapphire on NCL and they never allow flip flops, tank tops etc. in the specialty restaurants unless it is the Sushi bar. I will agree that NCL is the most lax when it comes to dress code but not what you describe. On Royal I wore shorts every night to the Dining room except formal nights, on Carnival I wore shorts every night except formal nights and even on Holland I could wear jeans to the dining room except on formal nights. Celebrity is the only cruise line that I would never dare to wear shorts to the dining room. With that said, the OP asked if t-shirts would be allowed. My response: if the Maitre D allows you in, you can wear whatever you want but if they refuse you, graciously change clothes and return. 🙂