Just be aware that the shops onboard will not be open until after the ship has sailed into international waters because they are not allowed to be open in port (any port). As long as you can wait until the first evening to obtain your autograph book, you'll be fine. If you have a kid who just "needs" to have an autograph each time he/she meets a character, it can be a long afternoon of waiting for the shops to open.
Enjoy your cruise!