As far as I am concerned the most romantic restaurant is Victoria & Albert's, but unless Wishes is late that night - you'll have a hard time eating dinner and then taking your cruise.
I believe the first seating is at 5:30 and it takes about 2.5 - 3 hours to eat.
A little bit more about V&A's
This restaurant has recently been reviewed and listed as a Mobile 4-star, AAA4-Diamond, and Zagat's Triple Crown-#1 in food, decor and service in Orlando. An elite, intimate, elegant retreat for world-class dining. Award winning wine list.
Well... if you want to go all out, and money is not an issue, you could charter the Grand 1 yacht at the Grand Floridian and have your romantic dinner on board. Or, you could set up a private dinner on the 3rd floor balcony of the Grand Floridian and then take your wishes cruise. My monorail restaurant suggestions are Victoria & Albert's, Citricos, or California Grill - in the wine room.
I"m going to suggest either California Grill or Citrico's. I would make a 6:00 ADR for either one that way you will have time to make your Wishs cruise.