I read this as well which is why I figured I would ask. The wording could be interrupted a few ways though I feel. "The only at-sea screening of Episode VII" could simply mean that
Disney cruise line is the only cruise line at sea showing the movie and it's not exclusive to just the special event cruises.
I guess unless Disney says something official, I won't know until we get on board. I am really surprised they are not doing a special Star Wars Day at Sea event for the day it premieres though! One of the reasons we picked these dates was because we hoped they would do something special for Star Wars Premiere. It's not going to ruin the whole trip if it doesn't happen, but I would be shocked if they don't show it. Of course we all know they are milking the franchise for any money they can. So much insane merchandise branded Star Wars (really Disney...shower heads?) and that $90-100 party event at HS is proof of that so maybe they won't show it because they don't want to lose out on our ticket sales. My only other thought as to why it won't be shown is because maybe they are being REALLY protective of the film to avoid any kind of leaks. Some very popular films in the past were not delivered to the theaters until the day before the premiere to keep things from leaking. Not sure if they would want the film sitting on the ship for nearly a week before it premiered.
Hopefully Disney says something official about it before we go so we can plan on having a REALLY late night!