Well, I have been on two Carnival cruises. I know there was a small laundry room that had a iron o use. Not sure if the is one on every floor or wing though. But there was one. I do not remember waiting to use it but only recall using it once. I generally have my clothes pressed for the trip.
They have an iron in the laundry rooms onboard the ship. Taking your own really isn't recommended because of the fire hazard. We always take a little travel steamer with us to get rid of wrinkles.
Take Downey Wrinkle Release. It works great on cotton tee shirts, polos, kakhi stuff, ect..And if you need the iron for Formal nights, just ask your cabin steward. Our cabin stewards have always brought the iron to us and then taken back after we use it. One suggestion, caryy the wrinkle release inside a ziplock baggie. It stains if it leaks. Happy sailing!!