We took a 2-pin Continental adaptor for our cruise on the Rhapsody, and had no trouble. Hope you enjoy the cruise: we've just come back from the Baltic cruise. We thought the food was very good: lots of choice and the staff were very friendly AND efficient! Cabin was about what we expected: adequate storage space : but take plenty of coat hangers! - especially skirt hangers if you need them. We found there were 3 'gala' nights, when you can dress up, but there was a LARGE variation in people's idea of formal wear. Take what you would wear to a fairly posh restaurant, and you'll be OK.
The wording in the brochure is: 110V/220V 60 cycles AC. Twin flat USA standard pins required. Assume this means US style adapters.
Not been on the Rhapsody but have been on other MSC ships (albeit newer ones) and they had both european and US plugs in the cabins.