Hi Sheena! Oh my, it sounds like you have a special occasion you will be celebrating at Walt Disney World. Your dress sounds beautiful and I can tell you can't wait to wear it on vacation.
Basically, all of the Disney
Restaurants are very casual. Some of the signature locations tend to find better dressed guests for the evening meals but often families will be coming directly from the parks to make their reservation time.
Victoria and Albert's in the Grand Floridian is the only restaurant that requires men to wear jackets and children under twelve are not permitted.
Narcoossees is described as a casual waterfront restaurant specializing in American coastal cuisine.
Restaurants that might offer a bit more of an elegant dining experience include Artists Point, California Grill, Citricos, Flying Fish, Monsieur Paul, and Morimotos. If you are set on Narcoossees then go for it and don't worry about your choice of evening wear. You will find an eclectic array with the guests and hopefully my own husband won't be there wearing his favorite frayed shorts he loves so much. I am determined to get rid of them sooner then later as I too enjoy dressing for dinner no matter where I am.