Cruise lines don't want electrical appliances brought aboard a ship for one reason---safety. They don't care about what power it would draw from the electrical system--they care about someone leaving something on ( drunks might just do that ) children being accidentally burned, etc.
As another poster said fire is the biggest threat to a ship at sea.
I retired from the insurance claims business and can tell you that many fires are started from people leaving electrical appliances plugged in, turned on, etc. You really won't use an iron that much--steam the clothes by hanging them in the bath with some hot water running-- hang them up carefully when you take them off-- use the spray that's available to remove wrinkles and lastly, there's a laundry service if you just have to have it. Several ways to get around having to try to iron and lastly, who wants to go on vacation and have to worry about ironing clothes ??