Back in 2010 one of the kids groups did that as an activity early-on in a 7 day cruise. I know they went around the ship looking for things, I had heard it was extremely popular. I can't say that I know it's there now. Perhaps this comment will help bump it to the front page so somebody who's been on a more recent magic cruise would be able to help you.
Another thing you could do is get a book of hidden Mickey's, I believe they have a section on cruise ships about hidden Mickey's.
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