To be honest, I wouldn't have anything, unless you really really want it. We did several cruises last year, and while the cake was very yummy (well it was very amazing!) I don't know if having food would be worth the cost.
For 8 of us to get Hot Chocolate it was $30. I asked about getting a cake again. (did I mention that the cake was incredible?) the base price was $40, if I wanted the same cake we got on the birthday cruise it would have been close to $80. I called about 4 days before we went on the cruise. If you are going to order something I would call now. I don't have the number, but it is Yacht Club private dinning. IF you just wanted dessert, you may my to look into the birthday option which is $100 more, then you would get snacks, drinks and a cake.
Mysunshine, make sure you dress warm that evening, the boats have blankets, but when we went last May, it was cold and most of us had shorts on. And if you have room, share with someone. The family that we shared with in May, was wonderful. We even meet up with them at Christmas and shared another cruise with them! It was great. We still keep in touch.
Have a great time, we will be arriving next week also, we will be there Thursday morning! Can't wait!