We were on the Wonder on the October 30, 2011 MR cruise.
They encouraged everyone to dress up and the main characters, Mickey, Minnie, Goofy, Donald were dressed up in Halloween costumes, and I think they had trick or treat stations for the kids.
They had tons of photo ops for everyone, had decorations up that night, and it was a lot of fun!! There was even a costume contest.
It was so much fun seeing everyone dressed up- Make sure to bring your costumes!!!
(Though I know some people just wore their pirate night stuff, it was still fun to see everyone dressed up in some sort of costume)