I bet there will be literally hundreds of copies of the Harry Potter book on the ship. I don't think anyone has yet really thought out the consequences of this. I can see it now...
It will begin with reports from the 7/18 10 night cruise, which will suffer a 80% cancellation rate as kids on that cruise realize that they're going to be at sea on Harry Potter release day and simply refuse to sail. A few lucky ones will get on our 7/28 cruise, where rates will suddenly tripple due to demand.
Excursions will be nearly empty on our cruise. The most common sound heard in the hallways will be "Mom! I don't want to see some stupid Roman ruins. Harry just found another Voldemort Horcrux inside a roll of toilet paper!"
Meanwhile, for the first time in
DCL history, adults will have free use of the now empty kids pool and slide, as the kids will be hiding in their cabins reading. Kids programming will be completely empty, except for the packed public readings of Deathly Hallows in the Walt Disney theatre, which will be hijacked by the kids programming staff (all normal shows being cancelled of course).
The Oceaneer's Lab will also be open to adults, if only to give the youth counselors something to do.
Personally, I'm quite looking forward to the unique opportunities presented by this situation. I will, of course, be immune to the Potter-Mania, as the previous Saturday morning will already have been, as usual, my traditional early morning drive to Costco to pull one off the giant palette of books they'll have. I won't sleep again until I'm done - which, depending on the length of the book, will be sometime that night.
This will put me (and others like me) in another unique position in the history of DCL. For if any kid annoys me in the slightest, all I'll have to do is look him or her in the eye, and tell them to buzz off or I'll tell them how it ends. Thet'll scare 'em off like nothing else could.
And even more fun! Inventing fake endings to really freak them out! Oh, I can already see that this is going to be fun...