Shops for cruises are few and far between, with very demanding reports, definitely not a good choice for a relatively inexperienced shopper. And I think there would probably be a lot of competition for the cruise shops. I have been told that cruise shops are offered to experienced shoppers with a good record with the mystery shopping company rather than posted on the job boards.
So, I guess the answer is to apply with a lot of companies, get experienced, and do an excellent job. Maybe, just maybe, someday a plum assignment might be offered.