The power supply on the ship is 115 volts and you need a US plug convertor. You need to check the specification of your straighteners to ensure they work at this voltage as well as the UK 240v standard (some models don't).
Check the plug on your GHD's as when I bought mine the assistant pointed out that the plug has a facility that allows you to remove a cover and then it becomes a 2 pin plug instead of the normal three pin. Failing that does as Bradley suggests and buy an adaptor. I am sure you will be able to buy the correct adapter on board. Hope you have lots of Good Hair Days!!
I was on the Star in April and was horrified to find my Ghd's did not work neither did my high powered hairdryer. I believe you can buy the duel voltage ones but mine are not and it seems the voltage on the ship is too low to run them.