I wore a nice casual style dress. I somehow forgot my nice sandals so I did wear flip flops but they had rhinestones on them so were a little nicer than the beach type. No problem and no one said a word. I did see another couple in front of us waiting to be seated and they were both wearing more beach like flip flops and capris pants (both were female). They were seated without issue. Seems like the staff doesn't bother looking down to see what your footwear is. I think it would be more obvious if a guy were wearing sandals.
Linda & Nick
CSA - Oct 2004 (for our wedding!!!)
CN - March 2006, Nov 2013
CSS - March 2013