There are meet-and-greet opportunities in the cruise terminal - however, there are *plenty* of times to meet the same characters for autographs later, so you certainly don't *need* to have a book before you get on board. In fact, I would recommend against pulling them out then even if you already have them - it's just one more thing to have to keep track of in the already crazy terminal!
The shops on board won't open until the first evening (they're not open in ports) so you might miss some of the first night character greetings - but again, very rarely are those characters unique and never seen again. Most of the greetings are princesses (usually out a couple of times per cruise) and the Fab 5 - who are out almost every night in different outfits. You'll see Peter Pan and Wendy, and occasionally some other random characters but usually later in the cruise.
Alternatively, you could just make your own autograph book. My DD10 has a couple of "official" books - usually purchased when we forgot our scrapbook paper!

What we typically did instead was cut 4x6 pieces of scrapbook paper and used stickers to decorate them for each character. We then slid them into a small portable photo album and took those with us to have signed. That way when we got home, we could add them - along with a picture of DD and the character - to a larger photo album or scrap book without having to cut up an autograph book - which usually has signatures on both sides of the page! Since I usually have tons of scrapbook paper lying around the house, it was pretty much free, and we always had one (unless we managed to forget, which happened a couple of times...)
DD10 has pretty much outgrown autographs - though she still loves getting her photos taken with the characters. She has had most of their autographs multiple times though, so that might be why. We'll probably take some paper with us on our upcoming cruises, just in case there's anyone we've missed, but really we all enjoy the photos so much more that it's not a big deal either way.
Enjoy the cruise!