That's true. Some lines don't permit irons. If you are on Carnival or Princess they have laundries on each floor. I know there were irons on board the Carnival ships I've sailed, and I think they were on Princess also. I try not to spend a lot of time in the laundry - at sea or at home! - but I did sneak down there on formal night to use the ironing boards for hubby's shirt. Then I discovered the wrinkle release and steamer.
Irons are not allowed as they are considered to be a fire hazzard.
Portable steamers work well, as does the wrinkle release spray, or hanging your clothes in the shower and turning on the hot steam. The ship may offer a pressing service for a small fee.