They have 2 pin 110v type sockets so you will need the standard international adaptor and the straigteners/iron will work just take a bit longer to heat up.
Apologies Costa Serena does have two pin 220-115 Volts.
Thankyou once again Blyth, you are a godsend!! It's good to be able to ask somone who does know what they are talking about.
It is not advisable to take a travel iron on a cruise ship,they are not allowed.
Fire is the worst possible thing that can happen at sea and travel irons are seen to be a hazard.