Lives4Disney, my guess is that you can eat on deck, it's just that the posts that I read were written by folks who all ate below deck. They were the ones who regretted not having more time to spend on deck during their cruises.
The person who is organizing the cruise that I'll be on stated today that she's got two desserts on order, one of which will be a surprise.
Hhhhmmmm... I wonder what that dessert will be?

She also mentioned that we'll have both non-alcoholic and alcoholic drinks while aboard.
*Hiccup!* It sounds like she's heavily into planning mode now. I'm sure our cruise will be fun (and very filling for our tummies).
Definitely arrange to have your cake served aboard your cruise. Let Private Dining and the Marina Staff know so that they can coordinate delivery of the cake for your cruise. And when you check into Citricos for your meal, make sure you let them know that you have the cruise booked for such-and-such time and that youll need the dinner check 30-minutes prior so that you can pay your bill and get your family out to the marina in time to catch your cruise. If the wait staff knows that youve got somewhere to go, they should be able to move your meal along accordingly.