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Hiya, my Wife took her GHD's on Milennium in June 2007 - They worked fine.. - Not sure if this helps.
I doubt that there would be a 220v supply. The voltage rating of your straighteners should be marked somewhere around where the power lead goes into them.Very often they can be 110/240 in which case they operate without problem. If they are just in the 220/240 range then they will take longer to warm up. Don't forget to take a plug adaptor to suit the USA style sockets.
The Constellation has 240v socket as well as 110v but they are usually marked, but if in doubt ask your stateroom attendant.
Whilst I have indicated there is a 220/240v socket in your stateroom taking the hair straighteners could lead to a risk of a fire, so best check with the cruise line to see if they allow you them on board.